Dave,
If you posted some more details about your boat such as displacement or planing, length at waterline, displacement (wieght) etc. I'm sure help will reveal itself
I'm not familiar with the Fjordling so not much help.
However......
My experience of 2 stroke ends with my last one, a 60 hp on a half ton 17ft wilson Flyer. At full chat 5000 + rpm and plaining at 23 knots she used appx 1 litre per mile. In a chop at displacement speed, you could easily double the fuel consumption. I used to ALWAYS carry 50 litres and resricted operations between Anvil Point to Southborne Rough after leaving Baiter in Poole. From mudeford, I ranged from North Head to the east, The Needles to the south and past Southborne to Bournemouth to the West. Mostly I had plenty to spare but opperated by going out and fishing on tank 'A' and returned on tank 'B'. I'd refuel both and go out on 'B' for the next trip keeping the rotation going. This in the main meant for me an adiquate rotation of fresh fuel and always a full tank to run home on, no matter what the conditions.
Bit long winded but I hope this helps.