In the spirit of Tip #3472, "Save and Reuse Multiple Offset Distance Values" and Tip #3873, "Perform Multiple Fillets with Radius Zero," Kent Cooper presents this similar routine that lets you save and reuse multiple fillet radius values with the F0 through F9 commands in AutoCAD.
This routine handles as many as 10 remembered radius values, but more commands can easily be added if needed, including the possibility of letters instead of numbers (e.g., FA, FB, etc.), as long as care is taken not to conflict with command aliases. The values are stored as environment variables, so they are retained even after exiting AutoCAD.
To get started, load the AutoLISP file. The command F0 will perform a fillet with radius 0.0. The first time you run one of the other commands, such as F1 or F2, you will be prompted for a radius. From that point on, the radius you enter for that command will be remembered.