If you've ever tried to use the Fillet command and received the message "Radius is too large," then this routine can help. This routine will report the maximum radius that will work in a given fillet situation.
After you load the AutoLISP file, run the command FM and pick the two lines to Fillet. The maximum radius will be shown as well as the current radius value assigned to the Fillet command in AutoCAD. From this point you can make your radius choice without worrying about using a value too large.