Calculating HO T5s and FC

[swords]

I'm going to try using the new High Output T5 lighting over my four new orchid & Nepenthes tanks and I was curious if there was a way to calculate how much light the setups put out in regards to FC? I know they're 5000 lumens each tube but how does that compare to the FC all the orchid literature always refers to.

I am looking at the 48" four tube and six tube fixtures. I don't want to burn the pleurothallids (which I love and will have many of) but don't want to have too little light for the higher light plants who can be mounted closer to the lights.

Top planted display tanks will be 48" L x 36" H x 24" W and lower greenhouse/potted plant tanks 48" L x 24" H x 24" W

Is 4 tubes enough over both sizes or should the taller tanks have the 6 tube fixtures?