Got a question for you experienced reloaders......?

Is there a rule of thumb for determining LOA. I just picked up some cast 150 grain gas checked/ lubed 6.5mm bullets to reload for my Swedishicon mauser. Here's a link for the ones I bought. Lyman #266673 140 gr. Can't find an exact recipe for 150's except jacketed bullets. I'm using Trail Boss powder. Trouble is if I seat the bullet to the top lubed ring (not the top ring) the round measures out to around 2.90 with the gas check almost to the bottom of the shoulder. And even though the bullet doesn't compress the powder, I was worried that too much gas could get beyong the gas check. On the other hand if I move the gas check to the top of the shoulder it looks like I've created a scud missile.

Suggestions?
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