Welcome aboard. We had a Precident 41 with SP225s. It would cruise at 10MPH and burh about 8GPH. If I slowed down it would get better economy.
With that hull and those engines, you are likely to find that you cruise at a consistent 7.5 knots +or- .5 knots. Your fuel consumption at those speeds will be 4gph again + or -. That will mean that you want to add fuel after 50 to 75 hours running time. Once you have done this for a season, you will be able to predict the no of gallons at fill time, based only on no of hours running time. After you get this data, no fuel gauges will be needed.