You can make between 2 unit-type there are 100 health points, 1 armor point and 10 damage points of difference.
If you don't the same improvement each time you can do like 90 health points, 2 armor points and 15 damage points of difference for 2 unit-type and 110 health points, 0 armor points and 20 damage points of difference.I hope you know what i mean
Or you can do every unit have the same armor type and weapon type. Anyway, i think the better way to balance is to test if it's not too difficult between two levels, it's just a feeling question.