How to find mean length per turn (MLT) for magnetic design from ferrite cores datasheet?
Look in the linked data sheet and, the first diagram of a core they give is this (with my red lines on it to show what an average turn might look like): -



Can you see that the diameter of that average turn could be calculated as: -

$$\dfrac{E}{2} +\dfrac{D}{2}$$

Or, if the core isn't fully utilized then use a smaller length but go no smaller than a diameter of D plus a little bit (former dependent) and, don't forget to multiply by \$\pi\$ to get circumference.