http://www.nacactfca.org/Johnston6stride.pdfUse the chart in this document.  Your mid mark should correspond to your grip height.  If you're on a 15ft pole, you're probably holding at 14'7", so your mid should be at 49'.  Once you dial it in, you might be up to a foot out from the prescribed mid 
at most, but any farther and you're doing something wrong, like carrying your pole too low or not planting correctly.  This is a really great tool for getting the run nailed down and consistent.