Once I've got any new setup how I want it and we have a decent run I always check the fuel used to get an idea for a particular outfit. I always go by the rule of thirds for what we carry... a third out... third back... and a third left in the tank as reserve.
I would expect the Elling may be a more efficient hull than my current Aerotec and much depends on the weight carried.
With 15hp 2-strokes in our typical use I've used much more than half a 25l tank so would have run out had I been using the Tohatsu's 12l tank.
Tohatsu do a comparitive guide but all figures are given for WOT... and for a mixed day out you will average far less revs.
As an aside one speed that is very heavy on fuel despite being slow is at displacement just before you get on the plane... say around 7/8kts.... while you push and drag a lump of water around.
For our 9.8 2-strokes Tohatsu say 4.9 lit/hr at 5500rpm. At those revs if kept constant you may cover about 28Nm and of course with the 12l tank run out soon after.
So other than a very modest trip I'd plan to carry the 5l spare and on a big holiday day out in Scotland carry the 10l spare. We do tend to use less fuel in the south of England with a SIB as our trip profiles tend to be shorter. In Scotland for example one place we like to stay it's almost 7Nm from the slipway to the open sea down a long sea loch so you have a 14Nm fuel use with that transit before exploring the open sea.
https://www.tohatsu.us/index.php?mai...%20more%20rows