Have tried 19, 20, 21 (A10 plunger)
21 = a real pain to use in town and traffic, acceleration negatively affected, but fine in 4th over 40mph.
19 = quick in town, good acceleration, but high revs in 4th.
20 = the compromise (currently fitted), somehow still a little too much clutch use to get going in 1st, but nice enough lower revs at speed.