I actually use acrylic which has a transmission of half that.
The ideal transmission percentage depends on a couple of things
What are you trying to achieve? If you want it to completely obscure the presence of a monitor, even 36% probably won't do that. I use the darker stuff for this, and the result is that you see nothing but the game display. The great thing about this, if you are using a horizontally oriented 27" 4:3 CRT, is that vertical games are very close to a 19" display, with no indication that it is pasted in the middle of a horizontal display. Horizontal games are obviously nice and large.
The other thing is what kind of display? If LCD, you may have difficulty getting things bright enough for the really dark material, without lighting up the background, which would spoil the effect. I think 36% would be fine for an LCD, but you'll probably see non-illuminated parts of the display through it.
Either way, you will probably need to boost the contrast and brightness, then tweak a bit to find what you like.