Edward, using your numbers on the lead alloy calculator gets me an answer of the following; Tin .14%, antimony .05%, arsenic .01%, lead 99.5% for a brinnel of 8.7.
Using 18 " of 1/8 wire lead free solder,(95% tin, 5% antimony), gets me 82 grains. The pile of shot in my palm,(#4 chilled), produces 3 ounces. I used those numbers to input to the calculator.
I'm not saying you're wrong, or what you're doing is wrong, as long as it works for you. The boolit fitting the bore and the use of a good lube is what's working. If, indeed, you're getting a brinnel of 18 to 20, there's some hardening agent in that lead pipe. The joints are usually 50-50 solder, that could raise the tin content, but not enough to account for the ten count of brinnel increase.
As for the OP, here's what the calculator said to mix for an approximate alloy brinnel number. 10 pounds of each will yield 2% tin, 6% antimony, lead 92%, for a brinnel of 14.7 .
12# of lead and 8# lino, gets you tin 1.6%, antimony 4.8%, lead 93.6% for a brinnel of 13.5. Both should work for good fill-out and a tough boolit that wouldn't shatter or be brittle. I'd go with the 10-10 alloy myself if that's what I had to work with.
The 12 lead to 8 lino would work also, but is a bit light on tin. If you could find a source for tin, sweeten it with .5% to make for better fill-out.
As always, boolit fit is king, the queen is a good lube. I use that boolit in my 40 S&W, with the STOCK Glock barrel , my M-22. I run it a lot harder 17-3 lino to pure lead. Has to be harder to eliminate leading in the stock-glock.