For those following along, today I re-pressure-tested the tank after using JB-Weld Silicon Gasket maker (the petroleum resistant type), along the top of the fuel-sender, and its screws, I gave it close to 36 hours to cure, it said it needed an hour. I didn’t want to rush it so was happy to leave it a full day + some before I re-tested, all of the previous pinhole leaks were addressed once identified (re-welded, not using JB-Weld!) and the JB weld worked perfectly around the fuel sender. So, with that, I was good to head down to the fuel station and to fill it up for the first time and to use this as a base volumetric measurement.
That’s video documented above, it’s 17.40 litres, 4.6 US gal. 
Specs on the C14: Fuel capacity 22 L (4.8 imp gal; 5.8 US gal)
So that’ll bring us nicely to a total of 10.4 US gal.
I routinely get 6.1L/100km from my bike, so that works out to be 645km from full to absolutely empty, so pretty much bang on 400miles. I was running a 5 US Gal tail dragger tank in the 2022 SBR, and was getting more than 400 miles per fill then. The C14’s range without an Aux Tank is fairly woeful really, so we’ll be glad for this for our riding in the USA.