I would do it at 300. I just did one with my 300 win at 250 and I found that it did not turn out as good as I expected because all of the shots were too close together. I worked up from 72gn to 74gn of RL22 with 210 bergers in .2gn intervals and 8 of the 11 shots were within .6" and the other 3 were spread around the outside of the group. I think if i was out a bit further it would have given me a better spread to make results a little more conclusive.