Crosstraining = Aptitude Malus of -5.3

[MainiacJoe]

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.

[Siegurt]

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.

Just to clarify, It is applying the -5.3 to the result of the training, the "pre aptitude" skill points, so you are actually always training the "freebie" skill at -5.3 apt compared to whatever training it directly would be.

In your ha example above, the 0.8 freebie skill for slings is as compared to training a +0 skill directly, not as compared to training slings directly.