How do you set the tension on a drive belt for a johndeere stx38 tractor?

The center-to-center spacing between the engine shaft and the jackshaft (i.e. spacing between the dead centers of the two bolt holes that will secure the two Comet pulleys) must be exact for a particular belt. The jackshaft will extend out farther than the Vanguard shaft so here is the most accurate way to measure the spacing: Put an accurate ruler against the back side of the Vanguard engine shaft and let it extend straight past the end of the jackshaft shaft. Then eyeball as accurately as possible to the exact center of the jackshaft and take the measurement. The Vanguard shaft is 1 inch in diameter, so add 1/2 inch to the measurement to get the true distance between the centers of the two shafts. It is hard to design and built a system and have the spacing come out exact, so some means of minor adjustment is helpful. Once this spacing is attained no further adjustment is ever made. The belt will appear extremely loose, but it will tightens up as the driver pulley increases RPM.