Another thing to keep in mind is the internal resistance of the meter you are using will have some effect.
e.g. if the circuit is as I have already described with a 1M resistor supplying the current to charge the sync. capacitor then if you had a meter which had an internal resistance of 1M, it would show a reading of half the actual voltage.
Modern meters have higher internal resistance and one with a value of 10M would show 90% of the actual voltage.
My old AVO 7 moving coil meter however has quite a low resistance. I don't know what it is but it is enough to fire the flash! (Vivitar 285).