Does that second bow move? If not that should be your height. Some move back and forth a little. In my Atlas chart it shows 21 1/2 with a plus or minus of a 1/4" Find center of rear bow and center of chrome strip. Measure out 18" each way from center and mark. Measure at these points. Measure from center of tack strip to edge of chrome. I usually cut a couple of wood to hold in place. You will find out later as you start to put top on why you need rear set right. There a couple of funky slits in top that must end up over tack strip. Mark center of top and work out each way. Good luck.