The choice is simple:
Either the water goes everywhere when filling, and you can't see the water level glass very well, or you make life extremely easy and fill your boiler using this MSS Loco Filler Sucker.
And then, when the days steaming is over, and the engine is hot, all you have to do is to remove the Safety Valve and suck out the unwanted water from the boiler.
The brass tube is the right length to reach the boiler bottom.
We use one in our workshop and boy, it is so convenient!