The HD-801 Fitting Guide is made with file-hard buttons having a radius of .242" for use when installing a Harrison Design grip safety. Be aware that while the guide buttons are "file-hard", you can still damage the surfaces if you grind against them, or otherwise abuse them.
Assemble the fitting guide through the thumb safety pivot shaft hole in your frame and use a sharp scribe to mark the radius on to each side of the frame tang. Remove the guide and use a belt sander to rapidly remove stock until you get close to the scribed line.
Once you have removed the excess frame tang stock down close to the scribe mark, reinstall the fitting guide and with the frame held in your vise, use a 6"-8" mill bastard file working across the frame tangs to take down the rough-cut tangs until you start touching the guide buttons.
Your tangs will now be within approximately .002" of finished radius on the tangs. From here, you will use the traditional "spot and file" technique to do the final fitting between the beavertail and the frame tangs.