Here's what I do for my 303's, not only for my Ruger #1A:
1) Neck up brass to .358";
2) Neck back down using neck-size die until you can just close the lever;
3) Square primer pockets;
4) De-bur flash holes;
5) Square case mouths;
6) Debur case mouths;
7) Separate brass by weight into lots (my lot of brass, however, all weighs within 1.8 grains of each other);
8) Use same load I use for my commercial BSA sporters, ie:
Lapua brass (alas, long gone, but I have 400 new ones on hand)

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WLR primers (seated by hand with Lee hand Auto-Prime)
41.5-43.5 grains Reloader 15. (I found 2 sweet spots, 41.7 gr. & 43.5 grain & am doing more development to see which is better)
Lapua 174 grain SP bullets (alas, again long since discontinued)
Seated to achieve OAL of 3.1" using a Sinclair bullet comparator
I haven't yet tried my trick of inserting a rubber o-ring around the forend screw & then reinstalling it.
I'm not sure what the point of stoking up the loads is, even though the Ruger #1 action is more than capable of moving up by ~10,000 cup /psi / whatever other form of pressure measurement you choose to use, as the bullets of .309"-.314" are all meant to expand properly at .303 British velocities, except for the 123 grain offerings, meant for the 7.62x39 velocities. Push them too much above those levels (ie. above their design envelope) & you're going to find the bullets don't behave as designed or wanted. You'll turn them into varmint killers that don't penetrate properly. Want to try something similar. Load a 170 grain bullet designed for the 30-30 into a 308 Winchester case, loaded to ~55,000 psi & do a penetration test. Now do the same penetration test with the same bullet loaded into a 30-30 at the normal 30-30 velocities & see which one penetrates further & doesn't have a jacket / core separation.
On a related note, Hornady has brought out ammunition / bullets for all of the other old Ruger offerings (450/400N.E., 405 W.C.F., 450 N.E., etc.). Maybe if enough people ask, they might recommence production of their old .312" 215 grain bullet, which was discontinued shortly before I started handloading (~1975).