I thought we had some of those calculators in the sticky thread Aerodynamic Tools and Calculators. I did a google for "servo load calculator" and turned up 4 sites right away with online calculators for immediate use. The links have been added in a post in the sticky thread.

Just be sure you provide a worst case speed of what the model will be able to do in a screaming death dive. And to prevent loading the servo you should use the number given by the calculator for servo torque required and get ones that provide 1.5 to 2 times that torque calc so you know the servo won't be operating near it's stall zone.

Enjoy!