Simple mathemagics to estimate maxes:
15RM * 1.2 = 10RM
10RM * 1.15 = 5RM
Of course, all calculations are estimates and if you want maximum results, you'll take the time to use the estimates to perform the actual lift to determine the correct max. The calc's work out to be pretty good. I suggest erring on the side of caution and round down when the calculated number falls between multiples of 5 (or whatever is the smallest increment you / your gym allows).
Hope that helps.