It doesn't matter what you put in for the stab, the MAC is for the wing and doesn't change. The one linked by OhD looks pretty good. I clicked the 'back' button on the bottom of the page and found the reasoning behind the tool. It makes sense and is inline with the article I linked above:

Basically the 25% MAC is about where the "Aerodynamic Center" of the wing is - a position on the wing in which the CG will generate no pitch forces (moments) regardless of the attitude of the wing.

This goes with the thought that changes in CG should not have an impact on elevator trim. If you change your CG and it affects your elevator trim, you are outside of the 'zone' - either too far forward or too far back. This also goes with the idea that the stab should not be generating lift for the airplane. The wing flies the plane, the stab guides the plane.