Hideho and good morning,

Some terrific replies thus far, we are lucky here to have such terrific information at our fingertips.
That said, I have a "method" that works for me, kind of a SWAG.
(seat-of-the-pants wild arse guess).
I use the pinch test to "simulate" the unloading of the prop.
While running on the table at the field, I shoot for max rpm.
Then, while at full throttle, I "pinch" the fuel line for a second as close to the NV as possible.
I listen for a increase in RPM, if I don't get it, I richen it up a click or two. Then I will from time to time lift the plane vertical and repeat the same procedure too.
This accomplishes two things for me.
As the fuel is used, and the tank empties, it compensates for the leaner run. When the prop "unloads" there is still enough mixture to go lean without going over-lean.
This is done without the use of a tach, or formula.
It has just been the way I always do it, and although clearly not scientifically accurate, works just fine to me. As Gene stated it's always trial and error, or trial and record.
I learned this method from a knowledgeable "old guy" years ago, (may he RIP).
BTW, this method rarely works in a cowled engine, although if you have access, a pair of needle nose pliers works too.

Nice thread,
Thanks to all who participated.