I got mine done to 13% transparency in dark charcoal tint and I've gotten TONS of compliments on it. with yours being blue it would look great with dark windows. The ransparency you got from the factory sucked; probably about 40-somethin percent transparency. I think I might have a pic of mine.. lemme sniff around...
The factory tint for early Mustangs was molded into the glass. They used a tinted plastic center ply instead of a clear one. You can identify this glass by the words Sun-X-Ray in the little logo on the glass. This type of glass is also known as "green glass" because that is the color it is tinted. It is a rather light tinting of only 5% maybe a little more. I don't have the exact level of the tint but I'm sure someone here will know or can dig up that info. The factory tint looks very nice but it is not going to reduce cabin temps too much and it you can see right through it. If you find some on a junker make sure to grab the vent windows as those are the ones everyone forgets about. I have a full set of this glass for my fastback that I pulled from a junk car and it looks nice in the car.
There ya go. That's 13% transparency, not 13% tint. There's a difference. 13% tint means it only blocks 13%. 13% transparency means it only lets 13% through!