When it comes to boiler-feed pumps, the two important variables are the amount of water in the operating range of the boiler and the steam-to-condensate time lag across the system.
The operating range is the vertical distance between the center of the gauge glass and the low-water cutoff point. It’s not something manufacturers usually publish in their literature, but if you ask, they’ll be able to tell you how much water sits between those two points. This is the only water you have to work with. It doesn’t matter how much water is in the entire boiler. All that matters to the feed pump is the water in the operating range.