Performance calculations for less than max. weight

Hi Everyone,
I was going through the performance graphs provided by JustFlight for the Piper Arrow III, and I wanted to calculate my climb and descend times and distances, fuel, but the graphs are only available for maximum weight.
How do you calculate climb and descend performances for any gross weight?
I searched on the internet, and looks like the full operations manual only covers maximum weight too, but maybe I'm wrong.
Also, what is a real world performance for descend? The manual's powered descend seems to be a little extreme, with 146 KTS and 1000 FPM. Is that realistic? If not, how would one calculate a more realistic descend's distance, time and fuel (also for any weight)?
Thanks,
Gabor.