Hey Downunder and Dar!
crankshaft induction timing is very-very important, though tuning is also very difficult. The cylinder porting is the rpm range mainstay, but if the crank induction angle does not match the rpm range, you will have one sick engine bugger at hand. I use carb blowback, visible as a slight fuel mist, to judge if I can let the induction last longer.
The induction range closing can be from 40° ATDC upwards. This is engine design dependant, so no hard figures can be given. If you goof, and increase the angle ATDC too much, low end torque will be gone, and you will end up with a lot of fuel blowback!