yep what Ppl. say ..
never use mirrors, don't reflect the right light, might only give you hot spots and burn your plants, like tin foil..
use Maylor cost so little to get them few sq. meters needed to cover the parts of the walls beside your light/plants ..
flat white is ok but according to my light meter not even half as effective as maylor, maylor is amazing and give me 3K foot candles pointing away from the light at the maylor wile I get 5K from the light, flat white gave me 1-2K .. all about 1,5 foot away from a 600W HPS
in your space I would go for 2x400W/600W Digital ballast that can run both MH and HPS
with a cool hood`s and run MH in Veg and HPS in flowering, connect them both to a nice 6" ventilation and a good carbon filter ..
and grow some kind of sea of green/scorg in there trying to use all the space
like this maybe:
fill all 4x10 feet with a screen of buds and let them two 600W spread there light out I bet that could work very well
if you have more space you should rather think about making a mother/Veg room instead, where you can start a new patch when you trow the fist in to flowering, so you can harvest every 2 months, that would really make a different and double your yield so to speak,
maybe use one 400W MH in there and some CLF for the mother, take the best plant of the fist patch and make a mother out of her, flower the rest, take clones of here and Veg em, if we say 1-2 weeks of rooting then 2-3 weeks as seedlings then there is around 4 weeks left to Veg if you grow a strain that takes 8 weeks to flower .. then you keep haveing small nice Veg plants ready for 12/12 under two 600W HPS
I bet with a littel practice and some experience you could Harvest 10 plants every 2th month and if each just yield 2 OZ then its 10 OZ a month ..