Re: How much water is in the boiler?

At SVRR this is what we are taught particularly with regard to the oil fired 19. With oil firing you can quickly make up for increased usage of steam and the consequent need to run the injector more often or longer. I always adjust the water valve for the minimum the injector will work with, without dumping a lot water on the ground, to reduce cold shock. I see others run the injector wide open but for a shorter time. I think it is better to run the injector longer at the smallest rate that will keep up with water usage as it would seem to me to cause the least stress on the boiler as you say.
Not being a coal fireman, I have to assume one can run into a similar situation with coal that we run into from time to time on the wood fired engine, which is due to holes in the fire, or wood quality. One can be losing steam pressure, while also using up water. If under these circumstances you were to shut the injector off after a 10lb drop in steam pressure which you are unable to recover from, you will soon find yourself with no water in glass. It has happened many times that with wet wood, you can have the firebox packed to the top of the door with wood that is barely burning, so you have to hope that it will dry out soon enough to start regaining steam pressure before you are forced to stop the train and build steam and also get the water back up to an acceptable level. It is better and safer to keep the water level up and hope you can get the steam back. If you are close to the depot stop, you can let the water drop trading water for steam as long as you time it right so you don't get too low. This is a good practice anyway so that you have plenty of room to add water once you have stopped to keep from popping off when the steam builds.
My 2 cents.