[biffbaffboff]

6 endurance = 0.183 per 1 energy
10 endurance = 0.228 per 1 energy
19 endurance = 0.482 per 1 energy
23.5 endurance = 0.701 per 1 energy
27.5 endurance = 1,170 per 1 energy
34.5 endurance = 1.357 per 1 energy

Most of this list was compiled using several results to get the mean average.
The results were for training in a cardboard box, no trainer with a Dstat.

The 8 level jump between 19 and 27 gave the best results, sadly the same ratio was not carried on afterwards.

19-27 gave a 2.427 x jump in trains from 0.48 to 1.17

27.5 to 34.5 the same 8 levels of end gave only a 1.16 increase in trains from 1.17 to 1.36

The credit cost for this increase to my mind is not worth the value of returns.

Hope this helps for endurance.