in such cases it can be helpful to have a plane in th part model that you can use to constrian at the assy level. consider creating a plane in your part at the mid point of your arc.
Also be sure you have "Peers" turned on to locate peer geometry at teh assy level.
What I do is make sketches that were created w/ the include command that maintains relations. If down the road you need to repair those relations, the include symbol gets deleted, and co-linear and other relations are used between one parts sketch and other parts edges.