There is a simpler method, see the attached workbench. - Calculate length of the arc - Divide that by two - Use the DistanceSnipper to shorten the arc by that distance. The endpoint of the arc is now logically the point on the centre of the original arc. - ArcStroker to make it possible to grab the coordinate with a CoordinateFetcher.
Ha! Now why didn't I think of that! ;P Oh, hold on... If neither of those work, try the following (someone else will have to fill in the correct commands here, but they all exist!) - grab the length of the object - divide by 2 (ExpressionEvaluator) - subtract that length from each end of the line - use PointOnLineOverlayer and CoordinateFetcher to add that as an attribute Alright, I'll shut up.
