How deep to go front to back

[kem]

It all depends on how tall is your tank. Try to keep the width close to that, and the tank will be visually pleasing. Too tall and the tank will look narrow, too wide it will look shallow (although some people like it that way).
The maximum width depends also on the material used to make the tank, and how thick the glass or acrylic panels need to be to keep structural integrity.