Ah, Alex, that's what comes from having "too many BSA's . . ."
Let's get to the truth, here, Richard. Detach the drive cable at the tacho, attach a small c-clamp to the cable end and rotate the engine forward one revolution. If you observe the amount of rotation of the c-clamp, you'll have the ratio - and also the direction of rotation which will tell you how the tacho needle will swing. Please let us know.
BTW - are you using a magnetic or a chrono?
David