They are ok after you get a technique, tap a tiny dot then make tick marks near so you don't lose it)Second, the slide scale is too short so depending a bit on headwind, this will not work for ultralight speeds(really anything much below 65 and the scale may fall out) nor performance aircraft (much over 250

Wind side has two major issues one is the type of plastic for the window doesn't take clear marks from most erasable pens and pencils; I have tried dry erase(green black red), #2 pencil, china markers(black grease pencil for glass and ceramics), and vis-a-vis wet erase