Hi,
Because they are turn off pistols, they have a fixed volume powder chamber. The ball size will be the diameter of the bore because the lead ball will make the gas seal with no need of a patch. James Lowe was a London maker so his guns almost certainly have London proofs on the breech or barrel. You have to measure the bore size to determine the ball size. Then, I would start with 2F black powder, fill the powder chamber half way with powder (probably about 10-15 grains) and fill the rest of the chamber with corn meal. Then put the ball on top of the powder and screw the barrel on. Prime and fire. Hopefully, you have the ring barrel wrench because the barrel will get a little tight after shooting a few rounds. As you gain confidence in the safety of the gun, eventually just fill the powder chamber with powder. Inspect the bore and breech before you shoot to make sure there is no heavy pitting from corrosion. Pay particular attention to the bottom of the powder chamber. Turn off pistols were very strong and very powerful given the fixed charge. After shooting, take the lock off, clean it in hot water with a tooth brush, spray with WD40 to disperse the water, and then wipe it clean and apply a little gun oil to the tumbler and sear and to the feather spring. Clean the barrel and breech the same way using patches for the bore and chamber, dry, spray WD40, wipe clean and lightly oil. Do not go heavy on the oil particularly the lock because it can soften the wood in the mortise. Also, before shooting check the trigger and lock mechanism to see everything functions safely. Look at the tumbler notches and nose of the sear for wear. Make and fit wooden flints in the cocks and dry fire to check the guns. Check that the guns cannot be fired at halfcock. Always close the frizzen before firing. Never fire it without the frizzen closed.
The greatest collector of British guns for all time, Keith Neal, repaired, cleaned, and shot almost all of his guns (thousands). He collected them to shoot them not to have wall decor.
dave