Work it out:

Need to know rear tyre circumference. Let's guess at 1.954m

MC28 Primary - 2.500
MC21 6th - 1.083
16/40 - 2.5

12,500rpm/2.5/1.08/2.5 = 1846.722rpm rear wheel

1 rev = 1.954m so that gives us 3608.5m per minute or 3.6kpm or 216.5kmh or 134mph.

So you're pretty close.

Interestingly I just worked it out for my 300 yesterday and it was hitting 130mph at the end of the chute. Pretty pleased with this as I'm running ancient Dunlop GPR's that are worn out and I was tippytoeing round the final corner so as not to highside. The Supersport 600's I used to spanner for were hitting 133mph at the same point back in 2000. Gotta be happy with that.