With racer tape on the top of the axle, we established its centerline and measured out from there by half the distance between the springs. Adding 7/8 inch brought our measurement to the center of each spring eye, as the springs are 1 3/4 inches wide. The tape was marked accordingly. Somewhat luckily this put the centerline of our axle perches exactly between two of the holes drilled through the axle. Though it would have been even luckier if it had aligned with one of the holes, as I could have used the Posies perches!