How much load? Center loaded or uniform?
This is the only place I go when designing shelves.
http://www.woodbin.com/calcs/sagulator.htm
A 50 pound (uniform) load on your size with .25 glass shows a .10 deflection (.09 would be noticeable) Bump to 5/16 and it is .05, 3/8 is only .03.
Personally with that span I would go with 5/16 at a minimum unless there was some kind of center support available.
Joe
ETA: uniform loading
JC Custom WoodWorks
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