Daniel, I presume you mean 2 miles/litre and not 2 litres/mile- yep Biggles got there b4 me. On our last valiant, we had the same setup as you (fuel wise) with 2 x 25l tanks under the jockey console with a 75 mercury(2 stroke). We also carried a small 5l tank under the bench. The one thing about the older tech (non-direct injection) 2-stroke engines is that they can be quite unpredictable as regards fuel consumption. What we did once to check our range was to drive as far as possible, varying the engine speed on one tank and then swapped to the second tank for the way back, we ended up running out of fuel with about 2miles to go (hence the 5l spare!!). We didn't account for tide in our test and this caught us out!!
You could always beef up the transom and fit a 75hp optimax. You'd get your fuel flow in litres/hour and also how many miles you're getting to the litre and your range taking into account how much fuel you have left. Oh and I almost forgot, plenty of off-boat excursions!!