It's virtually impossible to calculate the coverage an AP will give. The only sure fire method is a site survey. If you have a Cisco PCMCIA wireless card you can use the site survey utility that comes with it for this purpose. Setup your AP and do some walk around testing, trying to guess or making assumptions will be a stab in the dark at best and you may well end up with holes in your coverage.

If you want a more comprehensive survey tool then there are some very powerful (and expensive) tools on the market.