Using a 2x3 inch plexiglas sheet as a signal mirror at 0.7 miles

While the reflection of the sun from signal mirrors are amazingly bright, even the sun flash from a plain sheet of plexiglas, without any silvering, is very bright. Here I've taped the back of the plexiglas, so the only reflecting surface is the air-plastic interface in front, with a (computed) reflectance of about 4% at normal incidence. A regular 2"x3" glass signal mirror sun flash is visible to the naked eye at 24 miles on a clear day. Even if I assume the glass mirror is 100% reflective, using the inverse square law, the sun flash from the front face of this unsilvered 2"x3" plexiglass should be visible at 4.8 miles, and about 6.8 miles if I hadn't taped the back (which would allow reflections from both front and back surfaces). So - the fact that the flash is visible here should come as no surprise. I need to use a much smaller piece, or a much longer distance, to test my computations. To give you an idea of how bright it is - notice that when there's a white flash with the plexiglas, there's also a blue flash to my right, next ot the white stake to my right? That's the flash from my plexiglass bouncing off the inner pane of the double-pane window we are filming through, then off the outer pane of the double-pane window, then back into the camera! (With the actual mirror, in the second half of this video, you see a white flash were the blue flash was, and another blue flash - you see the flash after even more bounces!)