I made up a couple of plates from aluminum to attach to the ends of the log. The plates were squared up, and marked with rule marks in 1/8 inch increments. I leveled the plate in a vise, and epoxied a string level on each one near the top. Two flat head screws were put through the plats 9 inches apart. I cut some steel brake line for spacers between the nut and the plates. The plates are screwed into each end of the log equi-distance above the heart. Run a string line between the matching bolts, and set 3/8 or 1/2 lag bolts to the string line about 2 feet apart, then take the string line and end plates off. I had a bracket made up of two lengths of angle aluminum with flat bar welded between them with a distance of 10 inches between the angle legs. screw a good straight board or piece of plywood to the bracket to stiffen it and run the mill on. The first board I used, I melted paraffin into it to protect it from the weather and let the mill slide a little easier. I made my guide board 10 feet, but 6 to 8 is plenty if you put the lags 2 feet apart. The aluminum angle slides along the heads of the lag bolts. After making the first cut, I roll the log 90 degrees, and set up the end plates using a carpenter square.
I've cut timbers 60 feet long and over 1/2 a million feet of lumber using this set up .
Carl