I do something very similar to you, Romulus. Since a D-Day type invasion is a good move for the allies, it is safe to assume that if it is possible for them to do it, they will do it, and commit all their forces.
So I usually evaluate what will be left in WE (for example) what will be left after an all-out UK attack, taking into account that they may have slightly better than average luck. Then I see how what’s left will withstand a subsequent all-out attack by the US. The goal is to make sure that they either can’t attack, or that if they do attack and possibly win they will lose much more in IPCs than me. But preferrably I want them not to attack, because Germany can’t afford to lose units once Allied shipping is in full swing, and also, counter-attacking into WE moves my units in the wrong direction, away from Russia.
A pretty key part of my planning is looking at what will be left AFTER any battle that I’m considering, and then seeing how vulnerable it is - what can it be attacked with? Even if a battle looks like a winner for me, I won’t do it if the result is to leave 5 tanks next to a big pile of infantry, armor and fighters.
I will sometimes leave SE under-defended however. My thinking is that if I can lure the allies into landing there, it pulls 3-4 of their transports out of position. Thus for the next few turns, they lose the use of 24-32 IPCs of naval units as their transports move back to where they can be useful for other purposes, and fewer units are transported across the atlantic in the meantime.