The way I do it is spy on a guy, type his # of turrets into windows calculator, hit * (weap science) * (def bonus/gov factor) * 1.1 (random factor), then / 1.5 / (my weap science) / (gov factor) and bam. I never really felt the need to make a spreadsheet or anything..

if he's got def allies I just open a second calculator, add up their defense, then hit / 4, and then add that number to the his defense after multiplying in his bonuses, but before the 1.1 random factor. Only takes seconds.