As it is currently implemented, and with current values in the data:
Breaking grass height 1, gives you no seeds (1 net loss in seeds)
Breaking grass height 2, gives you 2 seeds (parity)
Breaking grass height 3, gives you 4 seeds (1 net gain in seeds)
Breaking grass height 4, gives you 6 seeds (2 net gain in seeds)
Breaking a grass block from the side/below (rather than removing the grass first) will drop the correct number of seeds too.
The one place where seeds will ‘not’ drop, is if you crush the grass, eg by placing a block of stone ontop of the grass which will remove the grass, but not drop seeds.