I usually do it by drawing lines out in the sand with my finger.
No but seriously.. If your using ford data and your extrapolating it for GM use.. you are making a ton of assumptions. The ford data doesn't cover GM pressure.
One screen shot you can see some of the FORD tables being fitted for use in GM. Another screen shot you can see the FORD data vs the best fit GM data. You can clearly see the pressure ranges barely overlap if at all.
To say it's easy.. well maybe.. just depends on how right you want to do it.
injector calcualtor.PNG
injector calcualtor2.PNG