there is only a point of diminishing returns when you can chose between 2 units, one that is cheaper per unit, the other is cheaper /combat die.
the formula 1, 1.4, 1,7, 2 is the steady state for combat between 2 types of units, and no mixing.
that is
12 inf v 6 ftrs -> 8 inf v 4 ftrs
so both sides has lost a third of its units, and the combat is a draw. when mixing units, there will be different results, and not as easy to calculate.
if you want the losses to be equal in IPC, then if inf attack v inf, you need 1.6 attacking inf per defending inf.
if attacking tanks that cost 5, you still only need about 1.7 to make sure losses are equal.