My spreadsheet for that was already linked upthread. http://www.telusplanet.net/public/kbetty/ruin.xlsBut he does provide something that can be plugged into Excel
What I would suggest, however, is something different (although iterative). Once a year, say, estimate how much income you will need for your "nut" above what is provided by current pensions, CPP, etc. The check annuity rates to estimate how much you would need to purchase an annuity to satisfy those needs. As long as your funds are well above that level, defer purchase of an annuity. Recall that the longer you wait, the higher the rate, and therefore the less capital required.