Take a tape measure and hold it along the bottom edge of the mirror. Reflect the sun's image onto a flat surface ( I've found that a darker surface works best). Measure from the sharp image to the front, silvered, surface of the mirror. That is the focal length within at least 1/4 inch or less, depending how accurate you see the sharpest image and how steady you hold everything.
Measure the height of your focuser fully racked in, add 3/4 inch. Add this to 6" (1/2 the tube diameter. Subtract this from the focal length. That will be the distance from the center of the focuser hole to the front edge of the mirror in its cell.
I would recommend that you place the mirror cell as far back as you can in the tube so that the adjustment bolts or nuts are just barely inside the plane of the tube end. That will give the maximum "lens hood" area at the front of the focuser.