you can use this formula - I keep an excel page open at the corner of my screen just for this - you plug in the points spent, the percentage achieved and it tells you how many more points you need to spend, ASSUMING THE WORST for rounding. It's never failed
=ROUNDUP((B3/(B4-0.5))*(100.5-B4),0)
B3 is the points spent so far, B4 is the percentage achieved