I ran ~13. Which I've since learned is higher than I needed; I left some pieces of the refractory support structure much more heavily scaled than they should have. Lower temp will have it last longer, I'm sure.
Today I did the same weld at about 10, which worked, sort-of. Other variables left me with a short unwelded section, which was a total PITA to close up, and in the end was part of what led to the piece getting dropped on the scrap pile.
Flux