Not really understanding your question. You use a comparitor instead of OAL (tip). Thats it.

You adjust the OAL based off the ogive instead of the tip, and note down the optimal length from the rim to the ogive as it says with your calibers with the comparitor. You set you seating down up to load the bullet to match that number and it doesn't change much.

Accuracy is not based off of tip, but when the bullet meets the rifling (ogive). If you go to the range and find 3.6" OAL for your 300RUM is optimal, and then setup your seater with the first bullet to reach 3.6", and then lock down you seater, you could go back thru the bullets and find they vary in OAL greatly. Your seater plug should be loading the bullet on the ogive not pushing on the tip.

But if you go to the range and find out that 3.85" according to your comparitor/calibers is optimal. Then you setup your first bullet to be 3.85 according to the comparitor, then go back thru all your loads, all should be 3.85"