nesting issue involving percentages

[statictwist]

I have tried an array of math to the following percentage issue to no avail. I have 4 cells. 1 is for positive credits B10 and one is for total pool A10. In this case the goal is 70%. I have a cell A51 which subtracts B10 from A10 to give the total negatives. B51 is the kicker. I need to use the information above to determine how many have to be added to both the credit and the pool to get 70%. First I decided to use IF statements ie; IF(B10/A10)>.69,""(IF((B10+1)/(A10+1)>.69,1,(IF((B10+2)/(A10+2)>.69,2,(IF....and so on...this seems like stone wheel days to me and there must be an easier way..It will only let me go to 7 due to nesting issues anyway. Anyone know the correct math? If I started with 5 credits and 12 in the total pool (41.7%) I know that I would have to add 12 to both the credit and total pool or 17/24 to get to 70%. I would like cell B51 to indicate 12 needed to meet goal...help