calculating overflow volume capacity

[revjlw]

I have an acrylic tank with built in overflow areas on each end. There is a 1/4" gap between the top of the overflow wall and the top of the tank AND there are 42 - 1/4" x 1" cutouts along the top of each overflow wall too. So, the question is, how do I calculate the amount of water that can flow through these spaces and into the overflow areas?