The number of wins so far (W)
The number of games so far (G)
The percentage of wins required (R)
And you want to know how many more wins (y) it will take you to reach the winning percentage, right?
100*W/G = Percentage wins so far
100*(W+y)/(G+y) = R
The y is your only unknown, and incidentally, the answer you'll be looking for...
100*(W+y) = R*(G+y)
100*W+ 100*y = R*G + R*y
100*W - R*G = R*y - 100*y
(100*W - R*G) = y*(R-100)
y = (100*W - R*G)/(R-100)
W,R,G are all known so you just sub them in and solve for y.
This assumes, of course, that you don't lose any more games...
Is that what you're looking for?
It sounds like an odd spreadsheet...