Tinfoil!!! That reminds me of Ricky from Trailer Park Boys. Get mylar, or if you really wanna spring for the good stuff, Orca grow film. It's more reflective than Mylar. Tinfoil is easy to crinkle, and will scatter the light every which way but loose. If you notice in the pictures on my grow log, the ones you just commented on? There's a wall of reflection in the back of the second one, and you can almost see my hand. I have a sheet of Mylar hanging on the back wall to help reflect light. If any of your friends grow, I guarantee they've got rolls of this stuff just lying around.

My humidity right now is 60% or so. Get a hygrometer. You can find them at a pet store in the reptile aisle. Perfect way to know how much humidity you have.

When he said 3rd or 4th set, he meant true leaves, not the leaves that came from the seed popping. The true leaves have between 3-9 'fingers', or so I call them. The ones at the very bottom, just above the seedling leaves, [at least on mine] have three fingers. Some have five. Most have seven, though. I've seen some bigger sativa's with 9, though.

Your CO2 system is much more advanced than mine. I go to the paintball store, have them fill a 20oz canister, and let it leak out in the room over night after I've sealed it up with tape. I also bought this CO2 generator that has mold spores that release CO2 as they decay the block they're in. It's got a one-way filter, so it's perfectly safe to keep in the grow room.