how much do i need to drain??

[IslandReefer]

When the center brace on my 90g came loose (last guy used the wrong silicone). With one end attached (to theback,in my case) I drained enough water untill the front pane had no bow in it. I just eye balled across the front, it was easy to see when it straightened. Then I attached the brace and held it tight with a clamp ( 8 bucks @ C.T., 36in long ) for 24 hr and presto...its held for 2 mounths now.
I did not fill the tank completely for a few more days because I wanted the silicone to fully cure and not contaminate the water with too much acetic acid, but with acrylic the dichloromethane (solvent) should evaporate completely in 24hr.
Good luck, hope my adventure helps yours