Water level on the slider is what percentage is fed to the boiler at, need to use the injectors to keep water in the boiler at a reasonable level, the WA box, unfamiliar with that loco but initially 60% seems to work for me in most cases then just keep an eye on the value shown in the WA box, if it drops a lot increase the injectors, too much like 100 shut them off until the value goes down to a reasonable level then just use the injectors to keep it there.
OK Pause for a bit and off to have a look.
Found the loco and ran a test started initially with 60% injector rate, and a value of 75 in WA.
I'm testing with out a consist on the back however it's been trundling around my test circuit for around 20 minutes at 60MPH, water at 72 injector currently at 9% to stop it dropping the water, was at 60% for the first 5 minutes or so before I backed it off as was getting over 80 in WA, obviously with a load it will need different settings.