I guess I am going to ressurect this thread from the dead, since I cannot seem to find a suitable answer on the forum archives, and the mathematical solution for this question works rather well. I am building an RV-8, but I am fairly certain that the empennage kits for both the 7 and the 8 are identical as they pertain to the installation of the electric trim servo.
As for what length to cut the threaded rod included in the trim servo kit, here was my solution, which is bit more precise than simply trimming and fitting and trimming some more:
My plans for the left elevator show a drawing with the trim tab in its level position on the elevator, with the trim servo and rod assembly also shown. If you look closely you can see where they drew in the presumed ends of the rod about half way inside each clevis. This appears to be right at the point where the clevis begins to change the direction of the flair in the shaft. In other words, about half way into the clevis shaft, but not all the way to the very end.
The drawing is in 3/8 scale. If we know the measurement of the distance of the rod as shown in the plans, and we know the scale factor of the drawing, we can obtain the full size measurement by applying the correct scale conversion factor to that measurement.
Measure the distance of the rod in the drawing. Mine measured approximately 1 3/8 inches.
1 3/8 inches is the same as 1.375 inches
3/8 scale is the same as .375 scale.
To get to full size from 3/8 scale you determine the inverse of the scale and apply that to the scale measurement from the plans.
So 8/3 is the inverse of 3/8, which equals a scale factor of 2.67.
Multiply 1.375 inches (length of rod in the plans) by 2.67, and you get a value of 3.67125, or just over 3 5/8 inches or so.
One other note about this is that when you check this measurement on your own airplane, make certain that the trim tab is in the level position, and the trim tab servo output arm is also at its level position. I checked this measurement against my assembly, and the measurement looks to be just about right.
This at least takes some of the guess work out of this process, and hopefully reduces the amount of trimming and fitting to a single event. Minor adjustments can be completed by screwing each clevis in or out the required amount.
I hope this helps somebody....