Do you have any info on how accurate the Cateye is? If the documentation doesn't specifically address this, I guess one way of getting a rough idea is what range of numbers you get for a given point over a series of days (making sure you have it properly set to begin with each time). I mean, if you calibrate it before starting and go to a point where you previously had a reading of 100 ft, maybe over 4 more days you get numbers ranging from 95 ft to 105 feet. Therefore, you could say it is within +/- 5 feet for that point anyway. More measurements give a better idea.