I'm getting ready to install some Toto Ultramaxs. Since these are low flow toilets and the purpose it to conserve water, I wanted to make sure I made the correct adjustments. How much water do we actually allow in the tank portion of the toilet? From my unlimited experience, when I used to replace these valves, I could fill the water almost to the top until it fell threw the "big tube". I want to make sure I have the correct height. Is there one? Thanks