I've noticed people mentioning one COB per square foot as a rough guideline. But as with everything else, don't consider that a hard & fast rule. If you're gonna run the COBs soft (my preference) you'll want to crowd 'em in a little bit.
A lot of people are bolting their COBs in place. I made mine so they can slide around on the rails, which is very helpful.
The little M3 screws will accept 1/8" X 1" fender washers. I bought a 100-count box at Home Depot.
Look at the flat face of your COBs. See the outer ring of screw holes? Screw four of the M3's, with one fender washer each, into that outer ring on the COBs. Place your rails so that the fender washers overlap the rail edges. This way the washers pinch the rail when you tighten the screws. I started out re-tightening the screws every time I moved a COB around, but soon realized there was no way they could fall off so I left the screws just a little bit loose.
Just one example of how handy this is. I started with four seeds. One never got past seedling stage (although I gave the poor little thing over a month to try) so it was down to three plants. I re-adjusted the COBs for the new reality without unbolting them and taking the risk of dropping anything.
Very good info! Thanks! I will be making my rails adjustable.