A while ago Jon mentioned in passing a way of ensuring that both elevator halves worked in unison by using two pieces of wood clamped, one on each half of the elevator and projecting past the rudder. If all is good they will be aligned and track together through the deflection of the control surface. This is probably a well established thing that everyone knew but me.
A bit late in the day I thought I'd check to see how it was looking on my Atlas, both halves are driven by the same servo, but via two push rods. I'd set it up by measuring the control horn lengths and ensured they were the same both sides and I used the wing tips to centre each half...
...I have to confess that I was staggered to discovered my two halves were not aligned, I then had to resort to utter shock when I discovered they didn't track together either as one side had a larger degree of travel than the other!
A little playing with the linkages and it's much better now.
So thank you Jon, it might take a while for your words of wisdom to filter through, but I usually get the hint in the end