My preference is around 5 degrees too.
To make it simpler to figure out, a 1" rise for each foot of run will get you in the ballpark.
If you put too much of an angle on the CP, you will have to crank your wrists to get them in line with the buttons, which will welcome carpal tunnel problems.
Along those lines, a CP that actually angles downward would be a great thing from an ergonomic standpoint.
They make keyboard trays for touch-typists that angle that way.
That keeps your wrists straight, which prevents your tendons from turning sharp angles, which is what causes carpal tunnel.