Most people seem to get roughly 1 NM/L from their ribs (almost regardless of engine and boat size) when cruising. Rough weather or a throttle happy skipper will use more (or if you are slower than everyone else and have to run full throttle to keep up).
People generally allow 20-30% spare fuel capacity, in case of miscalculation, delays, needing to divert to another port etc. So even if you do manage to cruise a 1 NM/L you won't have any spare capacity if your main tank is only 60L. I'd take a standard portable fuel tank (27L?) along as spare capacity.
In terms of working out the capacity of your fuel tank you can obviously run it till empty and then refill noting how much is required to fill it. A wooden stick dipped in the tank before and after will then show the levels of the full and empty tank (mark these by cutting notches not pen which will come off in petrol). Bear in mind that the tank may not be completely empty (and if you are only getting 60L out of a 100L tank this may well be the case) if (a) the pickup pipe doesn't go right to the bottom (b) the pickup pipe is at one end of the tank and the boat doesn't run flat. You can then measure off this stick on each trip (assuming your tank is reasonably "regular" in shape and get a more accurate idea of fuel consumption on each trip.