When installing my tail parts in arfs warbirds the easy part seems to be the incidence but the difficult part for me is the angle of the elavator to the rudder. As you know it should be a perfact 90 degrees but with a curved fuse at the tail its difficult to get that perfact angle when attaching for glue .
I do manage to get it by taping and measuring the angles , but do any of you have tips or techniques that are easy and accurate ?

one way is to cut away a part of the 90deg set-sqare (corner) then it reaches the fin & stab

or measure distances on both ends

That's what I would do, or miter a piece of basswood or poplar with 45 degree angles.

I also use a triangle with the corner cut out to align the stab and the fin. Another good check is, with the wing attached, step back and look at the stab over the wing. When everything is properly aligned the stab tips will appear to be equal distance from the wing surface.

Great tips , thanks