how do I?

[muskrat]

17.4 | 21.2 | 19.6

Add the 3 values. (58.2)
Divide by 3. (19.4)
Subtract from the nearest whole number to the next star. (20 - 19.4 = 0.6)
Multiply by 15. (9)

So, 9 skill points are required to add a star.

thank-you, this formula is what i was looking for.