Fastplane, when setting up pull/pull systems I think the most important factor is keeping the geometry equal. For instance, if your servo is off center and you come directly off the servo to the elevator than the distances are not equal and you will not have equal tension. When off center installs are neccessary they use tillers, which are free moving arms which have the pull/pull lines coming off to the control surface and are actuated buy control arms from the servo.

On the elevator end,If your horns are not centered over the hingeline than you will have slack on the non tension line. This is not a problem as Ernie stated unless you have a full flying elevator. This problem has a name it's called "ackerman".

I have split elevators on a 1/4 scale Fokker Eindecker and I use a servo for each with a tiller for each. Works for me now but I had the ackerman problem and in the neutral position the plane would porpoise. In up or down I had no problems because of "blowback".

It seems to me that you don't need top be off by much to have a tension problem.