I have been working on some compression and other things with the larger jig that you are using Stingray45. No two pieces of wood will give you the same spine, but each piece of wood will give you a comparable spine. It depends a lot on if it is sapwood, sapwood mixed with heart wood, or all heart wood. The larger the spine you are getting is not only heavier in grain weight but more so heartwood. And you will never gain spine in compression. On an average you will loose 2-5 lbs of spine per 1/64 you compress. I have done a few hundred doing testing and from what you describe, it has nothing to do with what size wood you start out with as squares, but rather the properties. I just did some form sap wood and the lowest spine came in at 44lb. The same wood, only heart wood came in at 70-80 lbs and these were all 23/64 out of the dowler compressed down to 11/32 and on every one I lost a min of 2 lbs spine to a max of 5 lb spine. You said you were running 3/8? What are you compressing them too? I have run a lot of 3/8 and have never gotten under 60 lbs yet. It has to be the lumber you are using
But on another note, compressing shaft you will not loose grain weight. Or at least mine have not.