Do I understand glass thickness?

I have a double wall, 2x6 construction with 2" between the plates. I will have two layers of 5/8 drywall on each outer face of the wall. So according to my book (Home Recording Build it like the pros) I will need 1/2" and 3/4" panes of glass for my control room? Is this overkill? Thanks

I think so - I reckon 1/2" (12mm) and 3/8" (10mm) are as thick as you need go.

It's just a matter af calculations. Your window/glass should, in mass, be at least equal to your the wall (GB). Since glass has a density about 3 times that of GB (2400 vs 700 aprox), the thickness needed for the glass is rougly 1/3 of the GB used. So, for 2x5/8 (30mm) you need atleast 10mm thick glass. So, yeah your assumption was right :)