Each year I take all of my LF lenses into a camera repair shop to have the shutters tested. I get a printout of each lens's actual shutterspeed for each marked shutterspeed. I put all of these actual/indicated shutterspeeds into the computer and come up with one cheat sheet that I print and laminate.

It's accurate and cheap since it only costs me $5 per lens to do this; so I think that it's not worth the time or effort to do it myself.