Thanks for the tip. Here is another method that I think I'm going to use: Take off the blade. Take it to a flat spot like your shop floor. Hold the blade down on the floor with your foot and mark the blade and floor with a black marker. Then roll the blade along the floor until the black mark touches the floor again and make another black mark on the floor. Measure between the two black marks and you have your radius and thus your blade length.
How did I ruin my blade? Cutting 30 year old blocks of Gaboon west African ebony. Really, really beautiful stuff. Totally jet black with no grain lines. In a way I hate ebony. Looks and feels really nice but absolutely destroys my steel tools. Not only is it hard as hell, but contains minerals too. Never again. I score this one as Ebony one point, luthier zero.