The .5 error could be attributed to using a different arm than the one the armboard was drilled for. It seems that Tri does not use any fixture when tightening down the mount. This does lead to vaariations from arm to arm. This is why Thom Mackris had me send him my Triplanar so that he could drill the Galibier armboard properly for the arm that will be used. I'm sure this is something Thom has learned from experience.
Best of luck!
Dan
Best of luck!
Dan