Ideally aim to start the turn after 600'QFE to be straight in by 400-500'QFE. On a 3 degree descent that should give you 1.5nm to touchdown, nicely inside the ATZ.
Of course 'ideally' here is based upon lots of assumptions to do with other traffic, ATC requests, noise sensitive areas, local controlled airspace, bad weather, etc. etc. etc.
I agree with HWD the most, though, once you are in the descent and on base leg on a visual approach, runway-airspeed-runway-airspeed is more important than watching the altimeter. It's best to learn early to judge how you're doing from the aspect rather than 'standard' circuits.