At 12" (30cm) off the tank, you'll get about 20" (50 cm) in diameter of coverage...which from AI's graphic....
...apears to be about 500-600 PAR in the center and around 200-300 PAR at the edges. You may have dark corners for the first several inches of tank space, but down further it won't be quite as bad. (Make sure that's the level of light you want....even stony corals should be happy - I have one tank I run at about 300 PAR peak - but I think most folks shoot for around double that. Know what to expect and you will be fine!)
Not sure it'd be for me, but doesn't seem like a bad option!!
BTW, all those numbers can be roughly converted to lux. Lux meters are very inexpensive, or even free in the case of apps for your smartphone. Get a meter to assist you, no matter what light you get. "Direct sunlight" at sea level = 100,000 lux = 2000 PAR ; 500 PAR = 25,000 lux.