Hi Involute,
Hand drawn image is not shown.

Draw a line 0.5" long from the top center of the existing arc, along the face.
Using 3pt arc, draw new arc selecting the bottom points of the existing arc, and the top of the line segment.

Selecting arcs shows their radii in the upper right corner. (Click Diam to switch to Radius display.)
Existing arc radius is 1.2725958"
New arc radius is 1.2660799"

- Brian

A point can be placed at the center of an arc.