The chart above gives some idea of how the weights change depending on alloy when using the same mold, and even after sizing.
Even if one doesn't have the exact mold or sizing die mentioned, one can extrapolate a bit to figure out what ones does have & uses will do depending on the type of alloy.
The chart does show that the "harder the BHN of the alloy, the less weight" example.
Hope these posts will be helpful in some way to folks.
I will make one suggestion to the OP.
Cast some pure lead ( or as close to it) and measure those bullets to set a "benchmark" for what you are looking for from those molds.
Already knowing what ya do about your 13.5 bhn results.
Then figuring out & knowing the difference between them in weight, compare to the chart, as well as even without the chart & using those weight numbers to average out what weight is acceptable to you & make "that" alloy by combining in the amounts you have figured.
Like making 10-1 Alloy with lead & tin, you can then do the same with your combination.
Maybe it will be 3 pounds lead(<what ever is your benchmark) to 7 pounds of your 13.5 alloy to make 10 pounds of the new alloy that meets your requirements, & so on.
Keep trying in small batches if ya like, just keep track of the ratio of "Bemchmark alloy" to your current 13.5 alloy, to use later for larger amounts.
OK enough typing for me for a while.
G'Luck~!