Assuming that the rate is added and not multiplied and that you have dura bonus, loaves would actually cause the tool to crumble twice as fast but if that cost matches the reward then go for it we got those too 
Maths: 100 - ( 50 + 40 ) = 10
100 - (50 + 30) = 20
Nevermind. I read that four times and I finally understood what I was missing. Ignore me. <3
don’t you know Prome3us Equation? 
Not op, but awesome. If you do the math, and keep buying the food, eventually you spend as much as just buying another tool. It is not a forever option, but a very nice one.
I assume these will be absolutely necessary when dealing with the expert tools.
All of the persisting foods give 30 min buffs…
The key here is to calculate what is cost effective. Bear with me for a minute or two while I take you through some theoretical math 
We all know persisting foods extend the lifespan of our tools. Many of us know (thanks to my earlier equation @boundmore) that your durability reduction skill has a large effect on how (and how well) these foods work. What most of us don’t really have a good grasp of, is what the actual value of the foods are to us as consumers.
I’m going to try and derive a theoretical equation here where you can input your specific variables and see what’s worth what at the end if the day.
Lets start by assuming a tool (gem or not) has b base durability. Your durability reduction - as shown on your character stats page - is d%. @boundmore summed it up nicely for us:
Soooo… This tells us how many actions or “swings” we get out of a tool based on our durability stats modifier. Before food buffs, for untrained characters, d= 0 and for fully trained characters d=40.
When we bring food into the mix, this increases d according to food quality. Lets add a variable “f” here, food buff.
The total equation would read
m’ = b/(1 - [d+f]/100) if d is taken as your skill reduction only.
Remember now that this modified durability only lasts as long as your food does, being 30 minutes.
To calculate the marginal value of your food, you must be able to determine the durability saving in those 30 mins compared to the expected durability loss without food buffs during those 30 mins.
This depends on the number of swings you can perform in 30mins, which in turn depends on both your agility with tools and the swing speed of the tool itself (iron vs copper for example, I’ll refine these numbers further in the future as I’m on holiday at the moment).
For argument’s sake, lets assume you perform s swings per minute. In 30 minutes, you perform 30 x s (“S”) swings.
To work out how much tool you have saved, we need to calculate the difference between unbuffed and buffed usage over these 30mins.
Unbuffed durability loss fraction = S / m
= S / {b/(1 - d/100)}
= S x {(1-d/100)/b}
=(S - Sd/100) / b
Buffed durability loss fraction = S / m’
= S / {b/(1 - [d+f]/100)}
= (S - S(d+f)/100) /b
Unbuffed - buffed loss
= ({S-Sd/100} - {S - Sd/100 - Sf/100})/b
= (S - Sd/100 - S +Sd/100 + Sf/100) /b
= Sf/100b
If my calculations are correct (and granted they were done on my mobile screen, please check) this means that the amount of tool you save in 30 minutes is directly related to swing speed and food buff strength (S,f), and inversely related to your items original durability (b)
It also means that you save Sf/100b % of your tool by buying and using foods. So the food’s worth is Sf/100b of your tool price. If you’re paying more, you’d be better off buying more tools instead of food. If you’re paying less, the food is cheaper than extra tools.
Nb- this assumes your tool lasts longer than 30mins without foods,
This doesn’t show the increase in durability, it just gives a percentage of tool saved - to calculate the added “value” due to persisting foods.
Please check me!
So technically we can drive the equation even further by adding dexterity as well huh 
Would it be cool if I attempt setting something up in excel tomorrow based around some of your equations?
Please do, I’m out in the wild with barely enough signal to post text, will only be at a pc again next Monday
that’s some science !
you have an eye for details - like that mining pattern 
Daimond Hammer with max DEX can swing 1.5 times per second.
So 30 min can swing about 30*60*1.5=2700 times
Exclude possible intervals (including rest) about 2500
Gem tool have 2000 Durability
Without skill you will broke hammer.
With skill you will get a half damaged hammer(2500/4000=62.5%).
With skill and Persisting Pie, your diamond hammer just lost 12.5% max durability.
62.5%-12.5%=50% => The Persisting Pie provides you this 40% save your tool 50% max durability.
The price of a diamond hammer is about 5000,
Equivalent to saving you the cost of the 2500c in tool side.
After deducting the price of Slyduda sold Persisting Pie.
2500-1099=1401
Obviously, this isn’t a losing deal
If your tools are not purchased but made by yourself, it also saves you a lot of time spent in collecting materials and craft them again - because your tool’s durability has become 10 times
Thanx for the example!!
As I said, I try to put an equation so it would work for anyone with any dexterity / durability reduction / any tool. The more swings you can fit into your 30mins, the more you save indeed.
Perhaps try the same example with a minumim dexterity bonus on a sapphire hammer/axe?
Also keep in mind that air strikes count against all tools. For a long time I didn’t realize that and ran around swinging at everything like a polygon with A.D.D. needing a shiny spinner.
The factors affecting in whether or not a loss you ate pie are only the number of times you have used the tool within the 30 minutes and the price of the tool .
Indeed!
We previously showed that foods have a greater % effect depending on your Base durability modifying skill, yet my math here shows profitability doesn’t depend on this skill but rather on dexterity alone.
I can’t double check my work above to confirm at this point in time, but it definitely seems that persisting foods work best with faster tools (makes sense when you think about it)
Please could someone post the mats required for Persisting Pie?
Thanks in advance!
Can’t test right now but maybe i was wrong with time lapse of the buff. In my mind it’s 1600s for loaf and 1800s for pie.
I think they’re all 30 mins duration.
Thanks for the mats link. Shame they require shimmering orbs, those things are so hard to collect more than a handful.
The Salty cuttle sells it as well! You can get there from the lava lavyrinth place.
Yep those orbs can be a pain… That’s why I stock the loaves 
also most other breads and stews/casseroles at almost cost