Once you have your rings installed on the action, take off the top ring half and you'll have the flat surface of the bottom half of the ring/base to put a small level:
Stabila 11995 Magnetic Pocket Level PRO with Holster
https://a.co/d/6pR3cVc
Set your rifle on sandbags/in a rest or cradle/in a vice and get your rifle level and keep it there. I set this up with a plumb bob down range so I don't have to move it again for the next steps.
Then install your scope loosely, and get it roughly level. I reference off the elevation knob cap or the flat spot on the bottom of the scope, using a metal ruler and a level. Tighten the ring cap screws a little to hold this.
Then, without moving the rifle, fine tune the scope leveling with the reticle and the plumb bob. Tighten it up and verify level one more time. If you didn't use loctite during that part of the process, replace one screw at a time, add loctite and torque to final setting.
That's how I've worked around setting up a round top receiver with no picatinny rail or other flat spots. I think Talley rings may have a small flat on them that could be used to verify that the rifle hasn't moved. Otherwise Wheeler sells a level that clamps on the barrel that could be set after you level the rifle and before you install the scope and lose the flats on the ring bases.