How many Robart Hinge points should I install on a control surface? X number per foot...One per X inch?

John, I make the decision based upon the amount of servo torque needed to move the surface. The more force on the control surface, the more hinges are needed. Also, I like to place hinges nearby and close to each side of control attachment(s). I don't have a formula like the one you are looking for, but the closest I've placed hinges (except for the ones near the control horn attachments) is 5 inches and that was on a 42 inch aileron moved by a single HS5955 servo. I've also placed hinges up to ten inches apart. If you have plans, then follow the designer's suggestions; but think about how to provide additional support hear the control horn attachment.

Fly safe, Glover