Float setting BS34 book says 18.6mm but

i want to use the clear tube method but what size or gauge of clear tube works best?

So it turns out that with my particular set of BS34's, the float height is supposed to be set at 22.4mm whereas my manual says 18.6mm. And after searching this forum was able to see some confusion as to what is correct. Clear tube check is the true blue way to be 100% sure. This problem is solved! Thanks to everyone here!