So this is how i think it should look like using simple math
a = talentlvl / maxtalentlvl (or just talentlvl / 5)
b = 1 - a
value = b*b*start + a*b*2*mid + a*a*end
*NOTE* just chainging mid value
using this we can get simple curves like logarithmic :

linear:

or exponential:
