As far as taking over a territory goes, doesn't it just require two things?:
- all defensive armies are killed
- at least 1 offensive army survives to take the territory
So on face you just need to meet two requirements:
ROUND(A * .6) >= D
ROUND(D * .7) <= (A-1)
Second one simplifies to (A-1) >= ROUND(D * .7), which becomes A >= ROUND(D * .7) + 1.
That second one's only relevant for D = 1, as (D - .5)/.6 < (1 - D * .7) doesn't work for any integer values of D above 1.
So we're really concerned by the first constraint, which becomes (D - .5)/.6 <= A, so you need to just subtract .5 armies (due to the straight rounding) and then divide by .6 (and round up since we're only working with integer values here).
Unless I missed something, it's pretty simple with a few adjustments.