HR and power can be related for an individual, within a narrow enough range. IMO, if you know your power levels for various heart rate zones, you could use a heart rate meter and work up an equation, integrate over time and arrive at total power. Then look at some of the online formulas and monkey with the guessed-at parameters until they agree with your hr-based estimate, and then use that parameterized formula for the duration. But a direct power measurement at least once is the only way to be really confident.

Or just pick a number between 400 and 650.