[JesseK]

What to calculate and Why
The banebots warning isn't about the motor load; it's about the physical load put on the shaft. Planetary gearboxes can bind if too much load is put on a shaft that's off by even a few hundredths of a degree -- and most FRC bots are off by that tiny amount. Thus, at 85 ft-lbs of torque on the output shaft, the gears inside will have a tendency to break.

How to calculate
The motor will simply put out enough power to push that torque as fast as possible, regardless of what that torque is (provided it's not above stall torque). To see what speed the motor will turn at a certain torque, use a motor curve (search CD-Media for "calc*" and you'll find several spreadsheets). So ...

Conclusion
If you have 43lbs at the end of a 2ft arm that's powered by a shoulder joint that has a P80 gearbox, you are putting 86ft-lbs of torque onto the output shaft of the gearbox. The Banebots warning is trying to tell you that this is a bad idea, regardless of what the actual motor and overall gear ratio is.