I do it in my head, or using a calculator (phone or computer or an old-fashioned hand-held). The goal is to figure out what the total number of FP added needs to be, so that the top unlocked spot is lockable at 1.9. The numbers needed are the total FP for the level, the FP reward for the top unlocked spot, and the FP added by whoever (if anyone) is in that spot. First I multiply the FP reward by 1.9, rounding up. Then I calculate total-1.9reward-1.9reward+spot.
So take the example of a CoA, level 11->12. Total FP is 631. The reward for 1st is 80. Let's say neighbor #1 added 9 FP, and neighbor #2 added 4 FP.
First, 80*1.9=152, no rounding needed. So to make 1st lockable, I need to add enough FP to make the total equal to 631-152-152+9 = 336. That makes 1st lockable at 1.9 (and 2nd will be lockable as well), and neither of your neighbors can do anything about it.
This works no matter who added FP, and no matter how much they added (unless of course they added enough to lock the position themselves). It works for whatever the top unlocked position is. It's really just a variation of the math needed for sniping, except that the goal is to make your building snipable at exactly 1.9 times the reward. ;-)