1. well the numbers are in range of 1-10 thousand (x).
2. well very large with O(x!) being some upper limit (for both numerator and denumerator) but final coefficient is ought to be small from 0.00001 to 10000 3. Yes absolute accuracy is a must have But I see that BrowserUK already had a similar problem when computing fisher's exact, so that mihgt be already solved problem.