With the Battle Calc, list the pu values (and total pu) next to the # of units.
So for instance, if you your on WWIIv3, two bombers (24) and a plane (10) summed at the bottom to (34). Attacking 2 destroyers (16) and a cruiser (12) summed to (34). The battle calc already says 1.08 units left. One could (mentally) subtract the price of the bomber, and say it gonna cost me 24 to do 34 of damage.
The PU's should adjust when adding and subtracting units.