That size of RIB and engine combo I would factor at least 1mile/litre as a ball part figure and then deduct 20% for a safety factor untill you get to know your RIB. So for a 200litre tank that would be a 200 mile maximum range less 40 miles....So go no further than 160 miles on a full tank.
One thing to remember is to gauge how much fuel gets pumped into the tank as some fuel tanks say 200 litres capacity but might hold less than this because of poor installation. My BWM would fill to the brim whilst filling on the water but when on the trailer the nose was higher and couldn't get full capacity because an air trap would develop at the front of the tank, if I wasn't carefull push the fuel suddenly up the filler hose with expected results.
Looks Slow but is Fast
Member of the ebay Blue RIB cover club.