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Hanno,

I don't know about the flat Bygrave, but with my cylindrical version (about the same size nearly as the original), I have done hundreds of reductions and rarely out more than 1-2' compared to calculator. the lat range seems to cover +/-nearly 90° as far as I can tell and for some reason i do not understand, why when you end up on the "compressed" side of the scales and are interpolating between fairly small scale marks, the accuracy stays the same. Maybe Gary knows why? (Interestingly, I have not found this with the Fuller, which is definitely more accurate in the "expanded" range of the scales. I get 4-7 places with the big Fuller depending on where you are in the scales)

Anyway, good luck. I'm sticking with cylindrical rules. I find them easier to make and use accurately using pre-manufactured, precision tubes, felt friction brakes, no pivot holes to drill etc. But each to their own I guess.

Best wof luck

Francis