They're about 35cm from the water's surface. As you can see from the first pic in this thread there is a "V" at the top in the centre, that's where the light doesn't reach with as much intensity as the rest of the tank (still reasonably well lit though). The right hand side light isn't quite as high as the left one because the holes in the metal frame wouldn't allow it, but I might try force them through after lights out tonight. That will make the V a bit smaller.
Any closer and it makes it a bit difficult I reckon. I planted some plants right at the back in the centre yesterday and got a hot cheek from the lights almost touching my face. But that's me being lazy and not turning the lights off or moving them.