From the Sierra 5th Ed manual:
I can cross-reference data from the Sierra manual for you if you post up what powder you're using.
My recommendation would be IMR4064, but IMR4895 or Varget would be good alternates.
Not sure what powder or what manual you're using for loading data but the Hodgdon online data center has data for the Hornady 150gr. http://data.hodgdon.com/cartridge_load.asp
The max COAL for the .303 is 3.075"...this is a general COAL measurement for magazine functioning, not because the bullet contacts the rifling. Anything longer than that and you'll have feeding problems from the magazine. You will have to seat a lot farther out to get a spitzer type bullet to contact the lands since the SMLEs have a rather long leade for the older 174gr bullet loadings which does have a longer profile than the 150gr bullets available.