okay, if you cover all the sides with reflective sheeting (mylar) and the front you leave just held down by a clamp or something of the sort you can still look at your plants daily by just lifting the 'flap' part of it. now for the second part, thats where the thermometer comes in handy. see how hot it gets at the plants surface at the given distance. so the ideal temperature would give you the ideal distance (while not exceeding 6-8 inches as that would decrease the intensity of the light once it reaches the plants leaves).
I had my tank covered completely and id only look at the plants from above and id take them out for pictures and such. its just a matter of how you would prefer to look at them, from a straight foward or from above/taking them out POV.