Mostly a Post-It In Case I Forget, or If You Are Attempting to Learn How to Cut CCW Arcs in gCode

 You want to cut a 90 degree arc where Xs,Ys in the outer part of the arc (the mill's edge will cut here; we want an arc cut (Xs-mr,Y-mr to (0,Ys)), where the center of the src will be 0,0 and the mill radius=mr. In this case, I am using a 1/4"  end mill.  The 1/8" endmill would waste less material, but it will not stay in endmill holder reliably.

Example code:

%
g17 g20 g54
g1 z1 f25
g1 x2.518 y0 f25  (position to Xs plus the radius of the mill)
g1 z0.1 f25
g1 z-.15 f1
G3 X0 Y2.518 I-2.518 j0 f5 (X and Y are the end of the arc)
(I is the offset from X start point to center of arc; J is offset from Y start point)
m2
%