Thanks Khoi, very helpfull!
Check with your glass shop again, but I think glass may come in just even metric intervals. Glasscages.com
lists 3/8 glass as 10mm thick.
Well, I just talked to the glass man and he said they work in inches down to 1/32. So, that should give me some room to play and set the panes flush with a little care not to press too far (and hopefully not have a huge gap left over). I plan on letting gravity and tape (just like you mentioned) help me with pressing and curing, rather than clamps. One large central bead, set pane and press to approx 1mm, then tape it down... sounds easy enough
I just hope black RTV isn't so thick that I need any mechanical assistance to get it pressed to ~1mm, if so I guess I'll cross that bridge when I get there.
I want to build a wooden jig to hold things in place and level while I drop the panes straight down, it'll likely take more planning than the tank itself.
My glass man just told me one of our local reef shops has one built up for their own builds, so I'll be stopping by there to get some more ideas on how to make this as controlled as possible.
As far as edging the bottom, I brought that up with him and he stated he's going to need to put a good polish on the edge to make it as flat as possible to maximize contact and bond, yada yada yada, so he's pretty much got that one figured out for me (they've been working on plans for building tanks commercially, and have a bit of knowledge here and there already
I haven't decided if I'm going to simply remove excess silicone or maybe mask and run a very thin bead down the corners then smooth it w/ my finger. If it's black sealant and the seam is small enough, then I guess you're not really going to notice it there as it blends in. I'm just not sure if it'll really help anything, but will probably do the bottom like yourself, at least, just to feel safe.
Thanks for the help so far, everyone! Anyone got some draw-ups or pictures of jigs to help me piece these images in my head together, feel free to share 'em.