On a composite airframe, is there a formula to correctly figure out where the slots go?

Thanks

There is, but the easy way is to roll the fuselage upside down and tape the arm to the outside bottom at the location it would be on the inside. Run the cables back to the rudder and mark where they would exit. The height of the slot will be determined by the height of the arm inside the fuse in relation to the rudder horns. It's pretty hard to get wrong this way.

Good idea. Thanks.