To find the degree of the arc:
2r*Pi= Circumfrence
Circumfrence/360=inches per degree
Length of required arc/inches per degree = degree of arc
So: Pick arc, then c or center, then pick center, then pick the start point or enter it as a relative cooridinate (@108,0), then enter A or angle to specify the angle, then enter the calculated angle (25.464791 deg).
I hope I made sense here.