This is my own personal experience, but I always err on the side of a stiffer spine. I had a shop tell me to go with a certain spine and with the tip, outsert and broadhead installed, everything came out to a little over 500 grains.
The first time I fired the thing, I could see it wobbling in the air on the way to the target. I went with a stiffer spine and that took care of it. They recommended 300, but I ended up buying the 250 spine (4mm Easton arrows, 29.5"DL, 76lb DW).
So mevertsen is right on. You'll need to take into consideration your arrow length and weight (when fully built). I cut my arrows so that they hang past the riser about 3/4" at full draw. Just personal preference.