No gauge and waterlogged tank what is a good pressure to set as a guess?

Why guess, use a tire gauge to measure the pressure in the tank.

Without a gauge, how are you going to set the pressure in the tank? You can set a compressor for 40# output and pressure it that way but you really need to put a gauge on the tank side of the pressure switch.

If the tank is waterlogged, then the bladder is shot and there is no point in trying to set the pressure. It needs to be replaced.

Assuming it's a bladder tank. I still have an old well on an upright 50 Gallon galvanized tank, no bladder, pump in air 2-3X/ year.

True, but in that case putting a set amount of air in the tank is not a big deal as in a bladder tank. I used to be in the same situation with my tank. Not too bad of a deal IF you can remember to recharge the tank well before the pump starts short-cycling.

Yep! Brian

Check the pressure on the well switch. The pressure in the well tank has to be 2psi below what the CUT-IN pressure is for your well switch. IE: If the well switch is 40-60 then the pressure tank should be set at 38psi. The well tank has to be entirely empty of water first to adjust the pre-charge pressure. Even if the bladder is bad doing this will buy you some time and wear and tear on the pump.
RJ

Use a tire gauge as hkstroud recommended to check the pressure.
RJ