Balance force components?

[low_bidder]

I don't do that by unit but by mission. Unless the terrain screams out for something else I use 40% in the first line, 30% in the second, 10% in reserve and 10% on each flank. The composition of each part depends on the plan. It doesn't make sense to attack thru a wooded swamp with armor. So my first line would be mostly infantry, MG's and small mortars. The second would be Armor plus mech, with the reserve being the same only less. Scouts on the flank reinforced with anti-tank kit.
SP is a combined arms game so I see it as a basic mistake to not use combined arms.