I did see that mylar has a thin film of metals (via vapor deposition), sometimes gold, aluminum, and other. The wiki article does say it can reflect up to 99%, which is why I was trying to get a number on the reflectors typically used on flashlights.
Its a pretty safe bet that unless someone is advertising it as something special, PVD aluminum was used on the reflector, as its cheap in bulk production runs, and the 99% applies to the reflector coating also, its just that, the more OP the reflector has, the more light that goes places it shouldnt (or is reflected multiple times and that 1% adds up over and over) and ends up being lost, in addition to losses in the lens.
A cnc lathe turned aluminum reflector vs a thermoformed (???) mylar film with the same coating should perform the same. With the aluminum reflector having the advantage of being thermally conductive (matters in stuff like P60 reflectors) and stronger, and more temperature resistant.
However, carve a reflector at home, apply spray glue, and line it with aluminized mylar, and it could be a decent start to a DIY reflector.
