Vertical Grow Light Requirements and Light Loss from ventilation tubing

Two part question here:

About to finish up 2nd vertical grow and I wanted to see what others feel is sufficient light, as I'm trying to decide whether or not to widen the vertical grow area. How wide do you feel a 1000 HPS can go...currently have a cylindar that is 4' wide, so the foilage is 2' away from the bulb in all directions, though I suppose at an angle up or down it is as much as 2.5' away (it isn't exact, but wire caging is used to control the foilage). I "feel" like the cylindar could be widened easily to 4.5' in diameter and there would still be plenty of light intensity with a 1,000...but I don't know, any thoughts?

Also, just started using vented tubing to channel the hot air out from the bulbs, but I don't know how much light loss might be caused by using such a tube...the light is traveling through addional glass (which always seems to be slightly dirty), and there is metal on the end of the one I have which is certainly robbing some light as well. Here is a link to the unit I'm referring to (the reflector has been removed so it's glass all they way around ecept the metal ring used to attach the ducting). I'm guessing there is maybe a 10% loss is light from such a unit...but again, I don't know?
http://www.htgsupply.com/Product-8in-Cool-Tube-Reflector.asp

Thanks,

I had a feeling I wouldn't get any interest/responses without pictures. I'll have to take some pics of the vertical opp and post them later.

The light loss isn't as bad as you think; with a flat piece of glass it's 1-2%, and (guessing here) with a curved glass it's gonna be more like 5-8%.

I wouldn't go much further than 2' away from the bulb if you could help it; you know better than us what your plants want, but you're starting to lose lotsa light when you back off 2'+.

I run an 18" radius around 4 stacked 600s which does me pretty good.

EDIT: I'd really recommend getting a light meter so you can know for certain what your plants are getting; I took some readings off of my lighting setup and it's nice to know what the intensity is and where.