Normally in my Non Magnum Encores I resize cases so they are 1/1000 shorter than the overall distance to the breech but in the 300 win mag caliber is it neccesary to resize till the belt rests on the chamber ridge? If I do this the case will be 8/1000 below the top of the barrel and 14/1000 difference to the breech.Any help would be appreciated.

Contact Mike Bellm, he can steer you in the right direction. He has a good article on his website about correctly sizing for headspace.

Bellm's TC

Resize to keep the same distance from the end of the brass to the breech face, .001-.002. The belt is not a locator, you still need to headspace on the shoulder.

I bought the RCBS precesion mic for the 300 win mag and set headspace off of shoulder. Measure fired brass and set resize die back 2 thousands. try a few resized brass in hte firearm before you add bullet to make sure the rifle will close.