Again, Tin (Sn) is not a good hardener. Antimony (Sb) is a good hardener. Use the tin to get your boolits to sharpen a bit more in the molds. Don't get hung up on the numbers. Your values are likely skewed for a number of reasons. The Lee tester is a good tester, as the method is similar to how Brinell himself tested hardness, but it is not a precision instrument. The force you apply to the lead will change the diamter. I went up to 200 lbs of force to get better measurements. I also went to a 0.25" indent, and should probably go bigger. I have trouble with BHN above 20 as the diameter gets really small. Also, my 0.25" indent tends to mushroom pure lead a bit, so this throws off the value some. I have a cabine tester as well, and get inconsistent measurements with it. The problem is the amount of force is too low.
The best you can do for hardness with Tin and Lead is BHN 17, which is 63% Tin and 37% Lead.
COWW's should already be BHN 10 - 11. They contain Arsenic, so they can be hardened by themselves. Water drop them out of the mold, or heat them the just under melting temp, and quench them is cold water. You can easily get a +2 to 3 BHN bump just by water dropping out of the mold. BHN's above 20 are possible by heat treating.
Pb/Sn Alloy does not take all that long to harden. I haven't studied that aspect much, but I would think a Pb/Sn alloy gets to it's hardest in just a couple days. Pewter can contain other elements, like Bismuth, Copper and Antimony, but is 92% to 98% Tin. Tin is expensive and a waste to use for hardening.
There is no set rule for hardening. Pure lead is hard as soon as it cools, and never gets harder. COWW's get harder by heat treating, and will be hardest when cooled. Pb/Sn alloys need a little more time. Pb/Sn/Sb alloys can take a month to harden, but it's not that simple either. Here is a chart someone did measuring hardness with professional equipment:
You don't say what hardness you need, but COWW's is usually good enough for most handguns.
If you need harder, heat treating COWW's is an inexpensive method.
Mixing Linotype and lead is better way to get harder alloy. Linotype by itself is about 21 BHN. If you mix 1# of linotype, and 1# of pure lead (BHN 5), you will get 2 #'s of BHN 13 (21+5)/2 = 13. I have made this alloy, and gotten BHN 14, which aged to BHN 15 in a month. I'm not using lab grade metals, and my measurement are only accurate to +/- 0.5 BHN, but I have gotten consistent readings, close to calculated values.
I was an engineer before retiring, and have wanted to get this BHN thing down myself, but still struggle with many aspects of it. I myself shoot .38, .357 and .45 at max loads with BHN 10 alloy, powder coated with Harbor Freight Red. So far this is the perfect mix for me.
A month ago, I mixed up 4:1 Pb/Linotype alloy. That ends up being 96.8% PB, 0.8% Sn, 2.4% Sb. According to the online calculator I should get BHN 11 with this mix. Here is my measurments +/- 0.5 BHN:
4:1 Pb/Lino Alloy
1 hour BHN 8.3
1 day BHN 9.8
1 week BHN 11.2
1 month BHN 13.3
Not exactly what the calculator says it should be.