[Jeffy]

1)It is a smart choice not to carry a compressor,
If you keep the pressure above 60 psi, the answer is simple and as you have explained. Once the pressure in the storage tanks goes below the regulator pressure, the relationship is no longer linear. I will let someone else get into it because I am not sure how to calculate it.

2) There are two kinds of motions you can get from pneumatics; linear or some curve using a linkage.

When designing, I like to make the best estimate I can as to how long a cylinder I can use. Then, I will often use one that is a bit longer.

When designing linearly remember that you can adjust how far the cylinder retracts easily by adding a spacer to the piston. Adjusting how far the cylinder extends takes a new mounting system.

For linkages:
Design the system so that you can mount the cylinder in many different places. A hole pattern spaced ~ .375 inches can do this nicely. And remember that you can restrict retraction with a spacer too.

One other note on linkages,
When the cylinder is perpendicular to the lever arm, you are generating the most torque. Anywhere less than perpendicular is generating less than the max possible torque. This is a good thing to keep in mind when you are designing clamps and wrist joints.