Basic Rules for Hardening Lead taken from rotometals.com

For every 1% additional tin, Brinell hardness increases 0.29

For every 1% additional antimony, Brinell hardness increases 0.92

Sample equation:

Brinell = 8.60 Brinell Pure Lead + (0.29 * Tin percentage) + (0.92 * Antimony percentage)

If you increase the amount of tin by 10% and the amount of antimony 5%, the brinell hardness of your lead will be 16.1

Brinell hardness 16.1 = 8.60 + (0.29 * 10) + (0.92 * 5)