3ft beckett? Play safe, leaves more extra capacity. You calculate the volume of the 3ft tower should be quite safe. You don't want to change sump again after you change something that caused more back flow right?
For me my new sump is catered for all my back flow and 4ft beckett capacity with another 30-40% extra capacity. That's abt 2x1.5x1.2ft space for backflow.