Crosstraining = Aptitude Malus of -5.3
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The skill points formula for aptitudes is adjusted_skill_points = 2 ^ (aptitude/4). Since crosstraining applies 40% of the earned skill points to the freebie skill, and since 0.4 = 2^(-5.3/4), and since multiplying is adding exponents, to find the effective aptitude from crosstraining subtract 5.3 from the species' normal aptitude in the crosstrained skill. For instance: Ha training Throwing is getting Slings for free at an effective aptitude of (+4 - 5.3) = -1.3 or 0.8 skill points. HO training Axes is getting Polearms for free at an effective aptitude of +1 - 5.3 = -4.3 or 0.47 skill points. Crosstraining doesn't affect the selected trained skill, the Ha is still getting 1x skill points in Throwing and the HO still 2^0.75=1.7x skill points in Axes.