Many folks start with full length shafts and shorten in increments, as buckeye suggests. That can sometimes lead to an arrow 2-4 inches or more longer than one's draw length. The extra length doesn't bother some folks, but I don't want any more arrow length than is absolutely necessary.
I start with a spine that's close to what I think is correct and cut them an inch longer than my draw length (so the shaft extends one inch beyond the riser at full draw, plus point) and tune with point weight changes. I agree that .600s should work for you regardless of how you tune them..