Flush with the Kaboom device in place. Let the tank fill. Note the height of the water in the bowl. Add water to the bowl from a pitcher etc. If the water level does not rise after the added water, the Kaboom is not blocking too much water.
I doubt that it matters, but there might be a tiny bit more water if you do your trimming on the output side of the cartridge instead of the inlet side, but the difference is probably not measurable.