advice sought on carb needle

["Evangelos G. Makris" <emakris@hol.gr>]

I have managed to find a used SU HS6 really cheap
and thinking of ordering the C-AHT771 inlet manifold
for the single HS6 and 1275 engine from England.

Can you please tell me which needle (letter code) is
the appropriate one for this application, assuming
"energetic" road use, an air filter in the K&N vein
and stock single box exhaust system?

Evangelos