Bending Stress On Rotating Shaft

I wouldn't think the inserted length is critical (as long as it is reasonable), I'd have thought the diameter and unsupported length was the limiting factor especially if the inserted length is equal to or greater than the diameter.
Note I have no formal mechanical design qualification, but a good dose of common sense :)
Del

I would suggest looking at thread specifications for this.

There is more detail required relating to material characteristics, intended thread form and the mating disc material

I suspect that provided it's significantly greater than the shear force of the pin, it will not matter.

Most valve shaft failures I've seen are sheared off, but I have seen some bent (in gate vale, not butterfly) where the tongue was fused in the closed position.

https://www.scribd.com/document/147032879/Butterfly-Calculation

http://people.eng.unimelb.edu.au/imarusic/proceedings/1/Tanner.pdf

This is typical of your past questions.

Just find an equivalent existing valve and copy it. That manufacturer will have known how to do the calculations you cannot.