Shrimpress you could still measure the deflection before you empty the the tank and measure the distance of the glass once it is drained. The glass is not going to retain any bowing from the water pressure. There also shouldn't be any noticeable change from stressing the silicone, that would be readily apparent with the tank full.
Did you have to hack the trim apart to get it off? I have a small tank that I got for free because it had been damaged during shipping. I want to relpace the glass and keep the trim so I can keep the lid. Thinking about using a heat gun to get it off.