The best way is to have an idea on your intended overall weight.

Start with the barrels and work back. I think you’ll find for smaller calibers that a wall thickness around the chamber of .200”-.225” tapering down to .100” wall thickness at mid barrels adequate. The bigger calibers need more wall thickness at the breach but still quickly taper down to .100”-.125”.

Barrels need to be roughly half the gun weight.

You’ll also notice double rifle actions have a wider action bar than shotguns. That’s why the fences on double rifles aren’t as pronounced as shotgun actions. This is done for strength and to get more weight between the hands.