Jim I will share my experiences of running two different models with the Cheddar ABC system. The first has the same boiler as yours that supplies steam to two Cheddar Puffins. It is from my steam tug that I have been learning from over the last 18 months. I set the boiler filling sensor to 2/3 full. On start up I top the boiler up to 3/4 full. This means everything gets warm and is running well before the water level drops and the system asks for water into the boiler, if not the system asks for water to the boiler very quickly, probably before you are ready to put the boat in the water and for the engines to be running and turning the pumps. Also the pumps need to be turning over at a reasonable speed to catch up with the water used.Once on the water and running the system keeps up with demand, maintaining the boiler at 2/3, for 40 minutes or more. The oilers have not run out of oil during this time, the oil seperator does fill and is the limiting factor. I use straight steam oil in the oiler and have been using steam oil for lubricating the engine bearings, steam glands and moving faces.
The second is the same set up as you have, again I fill the boiler to 2/3 and the system has no problem keeping up with demand, the engine revs faster than the puffins and I need to work further on the settings for the boiler feedwater bypass settings since at higher revs the boiler level rises too far. I have been impressed at the revs of the Proteus engine and the ability of the set up to maintain the water level in the boiler at about half revs (2000rpm) while also maintaining 60 psi without noticeably dropping the pressure or revs when the pump is operating to fill the boiler. it just keeps going,
What I have learned is that the system can and does maintain the boilers water level at 2/3 and it does this better at higher rpm, I do still bring the model back in to check all is operating as it should. Whilst not directly answering your questions perhaps you can use these observations as a starting point for your own experiments since what I am finding is that each steam plant had its own character, so what works for one may not for another. hope this helps, Dave