I have come across an inductor calculator for the zobel network on this forum way back. Life is always like that, when you don't need it, it's flashing up here and there, but when you need it, it's nowhere to be found. Does anyone remember where to find it?
The length will be reduced if you increase the diameter of the coil( because the inductance is proportional to the product L*d) and if you have little spacing between turns.
For 20 mm diameter and spacing of 0.5 mm i get 6 cm length
Interval between turns seems to be the distance between the edges of each turn. So if it is a close wound coil the distance is '0'.
Try 'several layers' selection on top. You will get a two layer answer with length of your choice ( 25mm? ).