My memory is that the clip make little sense. It had teeth and sits inside the channel, as I recall you could insert it one of two ways, it basically prevents movement one direction. So if you put it in one way, it will go up easily, but not down. Or it can go down easily, but not up.
So I put it to prevent it from sliding down. Two holes in the sump, mounted such that with 8-12" height above tank it will float above the ground. The top + light screws in, there is a piece to hide the cable.
If you have a reefer 170/250, the glass overhangs the back slightly, so you need the washers as I had. If you have a reefer 425 (thicker glass), then it's flush with those stands and you don't need the washers. I had to buy a bunch of washers to prevent it from hitting the glass.
At 12", you get about 385 PAR at the surface. Depending how much power + spread you need. Mine is currently 8" above, so it's unlikely it will touch the floor.
On the vertical piece, the top has the two holes for the crossbar. I remember putting the clip in backwards and having it to fish it back out because I had it backwards.
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