If it has to be SAAMI, use a headspace gauge. If for yourself, a fired case will do... Screw the barrel on 'till it bottoms on the gauge or case. If you're using a fired case & you want to be fussy, add .001 (back the barrel off a reasonable amount for head clearance). Now measure how much space you have between the receiver ring and the barrel shoulder. Measure the lug, subtract that from the first measurement, and that's the shim you need. Yeah, you could add .0004 for crush if you're being fussy, but unless you're grinding down an existing oversize lug...
As a "but first," I'd use either the gauge or the case-in-the-barrel just to check that the area above the web of the case is in fact supported by the barrel. That's a safety issue; unless you know for sure, no reason to just guess someone else's work was safe...
A whole lot depends on whether or not the rifle's for you. You can always fireform brass to fit just about anything that will still work with your dies. If it's for someone else, make sure factory rounds chamber and fire safely.
EDIT:
In other words, it's kinda like putting a barrel on a Savage, but instead of just turning the nut, you have to make a shim.