How many bulbs will also play a role. A 2 bulb unit is probably about 18-20" and 4 bulb more than that. You're height will probably burn some phals, but tough to say.
I have no idea what kind of light you get with this kind of fixture without a reflector. You should try and measure the light levels with a meter, if someone doesn't use this and know what kind of light they offer.
I have these exact fixtures although I always make shades for them. I grow some of my Phals 8 to 12 inches under 1 of these with a shade to reflect all of the light on the plants. They are fine. Just introduce them slowly if they had a lot less light before and keep an eye on them. Some of the ones that have pink or purple flowers get purple pigment in the leaves but that is fine.
Thanks! for 3-4 days i just put them on 30mins -1 hr at a time 4-5 times a day, they seemed to adjust well. Ill pick up a reflector for it when i get a chance.
Sounds like a good plan. The same site sells reflectors but they are a bit costly. I just make my own by covering some cardboard with mylar (I buy a cheap survival blanket from places like Canadian tire) and it reflects really well.