Owen Duffy (VK1OD) has produced yet another useful calculator on his website. This calculator makes it very easy indeed to evaluate power levels of your radio gear with your oscilloscope. Section 7 in 'Experimental Methods in RF Design' details measuring your power with a 'scope and provides the formulas you need. However, with this calculator you can make your measurements with your scope, and then just plug the numbers in and get your answer – couldn't be easier.
Enjoy! Thanks Owen!