Essentially you want to use the minimum amount possible (as APAP dissolves a little bit in water) without it being thick and goopy as that will just clog your filter..
The CWE megathread suggests 40ml per 20 tablets and also lists the solubility of APAP, ibuprofen and aspirin. With 7 tablets you wouldn't need that much but you don't want to use too small an amount as some gets absorbed by the filter and you'll lose a lot of hydrocodone if the volume is tiny (therefore the concentration is higher).