Thanks all for the excellent responses. I had not thought of the issue of a marker on a glass getting removed in future, leaving an unsafely designed glass.
At this point, I am planning a lamont for my next boiler, so at least there won't be any crown sheet issues - the water level will be in an unfired vessel. I suppose the lowest safe water level would be enough water for the circulation pump to work reliably.
Cheers,
Scott