OB Shaft length (again)

[Mr.H]

As so many before me, I need to consider buying a new motor for my 22FK, and wonder about the length of the shaft.
I did dig in the old posts, but could not find a satisfying answer.
Apparently most use a 25" shaft, but everything also depends on the motor mount.

So my question is: where on the hull do I measure for the right shaft length?
Do I use the lowest point on the stern or do I use the lowest point of the hull (minus the keel)?
Or something else?

Thanks for once again dealing with OB shafts!