Hi Arne, hi Steven,
I'm an owner of a JR Kingfisher 20+ and the mast on my boat is situated at the fore end of the coachroof. It looks so also in Mimpi (the German one) and in Pilmer. I'm not an expert but the mast in the position that you suggest would stay approximately at the convergence between of the two fore bunks and the sail catcher. I don't know if this would be a good position to bild the mast foot.
For the sail I've used the sail plan of Froken Sørensen (SA 20,4 sq.m.). The reason way I used the plains of a sail of another boat is that I've made the sail myself without any prior experience and I wanted to avoid miscalculations while drawing the sail myself. They sail plans of Arne are extreme detailed...so I just grabbed one, let it print on paper and cut the panels!
I don't know how will perform the boat with the new sail. I plan to have the first trials in summer in the gulf of Cagliari in south Sardegna where I come from. About the making of the making of the sail: I can just reccomand the scripts of Arne (on this site: Junk Information; Public domain files by....; Arne Kverneland). They give clear explanation of every single step and are plenty with pictures of different sails making and rigging and more...(thank you a lot Arne!!).
I've hoisted the sail a couple of weeks ago. That day it was getting too dark and cold to install the Hong Kong parrels. On that day I've installed the lines for the throat hauling parrels and the yard hauling parrels but just for the optic because I did'n have the blocks for them. I think that that's way there are a lot of diagonal creases! Meanwhile I have the new blocks and I'm ready for the HK parrels, but I'm in quarantine and I cannot go to the boat.
If you want, I can take some pictures of mast, partners and mast foot as I get to the boat again to complete the rigging…
Ciao Mauro