I have a tree that i have tested with my refractometer and found the sugar content anywhere from 1 to 3 percent depending on the weather and year and somtimes just the freeze/thaw cycle. Im more inclined to pick trees due to there health. If the tree has 4 leaves on it in the summer or hollow trunk or physical damage then i skip it. If your going to test the trees maybe just test the questionable ones. Your going to have to drill them to check them anyway so you might as well drive a spile in.