How much light is reflected by a glass cover?

["Pete" <peteal at sympatico_ca>]

Hello, I had a glass cover for my 90gal tank for evaporation and jumping
fish protection for a long time.  Recently I'v tried it without one as I no
longer have fish that are a high risk of bailing from the tank.

I now have noticed that my Rotala rotundifolia is now getting red at first
1/3 of the tank bottom instead of at the top 1/4 (where I'd have to soon
prune it anyways) and other plants are bushier.  I have two 175W MH lamps
pretty close to the surface so I thought with 350W of MH I had plenty of
power to get light past the glass cover but since I see an improvement
without the glass I guess not?!

So does anyone know about how much light I was losing with this cover
reflecting light from the tank?

The MH lamps have a tempered glass shield also... the cover was more for
fish and tank protection.

Thanks
Pete.