Factory tint %???

Yes they were. All fastbacks had their rear windows tinted, without exception. The options for tinted glass were; windshield only ( it was actually the shaded style), or all windows which actually were tinted green and called SunRay, except as noted for the windshield. The percentage call would be a WAG at best.

65 Hi-Po F/B
67 GTA F/B (small block)
MCA 27479

I heard that all factory A/C cars in '65 automatically got the tinted glass, but I'm not sure if that's correct.

The poster formerly known as JAV65

That's me on the left...
http://albums.photopoint.com/j/View?u=1572917&a=11954864&p=43026383.jpg

Average tint on today's cars is 70 %. This means 70% light is being transmitted through the window. As far as you application I'm unsure.

66 Signal Flare Red Vert
302 2V