I agree with 1.6km per litre as a sensible planning figure. If it is a modern four stroke (presumably the Johnson equivalent model to the Suzuki DF115) then you'll probably improve on that as my 150 E-TEC does 1.15nm/L in the cruise (2.1km/L) and I would think yours should be better than that.
The E-TEC engine gauges give you a running readout of fuel economy at the current speed, does the instrumentation on your engine offer anything like that? I found it very useful in finding the sweet spot for economy on my boat, which is about 25-27 knots.
I think falcon meant you should work on a 20% reserve over and above what you expect to use but maybe his percent key was broken
So if you worked on a useful range of 100km on a tank + 20km as a reserve that would be sensible, until you get a better feel for the actual consumption which I suspect may be better than that.