Vivarium lighting with T5's

[Turtle81]

OK, I have spent the last week pouring over these forums and others tying to work out how many 8Watt (12inch) I will need for my new Exo Terra Natural Terrarium (45 x 45 x 45 cm / 18” x 18” x 18” (WxDxH)) and I think I’m more confused than when I started ! I would use 13W t5’s except they are too long and overhang the edges by about 3-4 inches.

I plan on growing my small cattleyas in it, I currently have:
Ctna Dal’s Capri x Bc Dal’s Charm
Pot Dal’s Emperor x C Jerome Shultz
Blc. Momilani Rainbow x Slc Red Jewel
Slc Jungle Treasure x Blc Waianae Leopard
Slc Pink Love x Slc Seagulls Mini – Catt Heaven

They will most probably be 9 inches at the furthest point away from the light fitting; I just wonder if I need 2, 3 or 4 8W T5's to provide the required amount of light as I know they need a fair amount of light and they won’t get much outside light either. I'm currently thinking 3 or 4 bulbs?

I just don’t want to spend the money to find out I should have bought a larger fitting! .... or a different type of light?

Any help/ideas would be most helpfull and ill try to keep you updated in its progress although it will be a slow process due to $$$