I want to measure the highest point (midpoint) from a flat line of an Arc between 2 Fixed Distance Points (340mm apart) (as below).

I assumed it would be extremely simple, but Solidworks wont let me do it.

I want to measure the buckling deformation height (due to Linear Expansion) of a material pinned at 2 fixed points, but when I try to change the Arc length dimension to 340.3mm (from the arbitrary dragged value) it says "changing this would result in invalid geometry", which should not be the case...?!