This routine allows you to select an existing curve to set the radius for an immediate fillet in AutoCAD. This is similar to Tip #1468 except that Kent's version here also handles polyline arc segments in addition to arcs and circles to set the desired radius.
After you load the AutoLISP file, run the command FMR. Select a curve, and then select two lines to fillet. The fillet radius will match the radius of the pre-selected curve.