bonjour2, first of all. You intend to place ur sump in the cabinet or you are using wrought iron 2 tier stand? I dun know anything about calculations but I can tell you what I did to my system before.
Try to custom made your sump tank big enough to go in your cabinet if you are having a cabinet. I had my sump tank made at 2.5ftx1ftx1ft to nicely fit into my cabinet.
I had my main tank 3ftx1.5ftx1.5ft drilled 1 25mm as overflow & 1 20mm as return with a glass compartment. No problem encountered. (Without a glass compartment is rather dangerous. Live stock might get suck in or the overflow pvc hole got stuck?)
Like all bros said. There's no too big or too small for the sump tank. I believe it's up to you what you gonna to have in ur sump. Your setup quite similar to mine. I made the sump tank with 3 compartments:
1st compartment, 4" length for mechcanical filter.
2nd compartment, 16" length for DSB compartment.
3rd compartment, 10" length for holding my weipro 2012 & 2 pumps.
Go for a 3ft sump tank if you are using wrought iron 2 tier stand. (Just my opinion)
Hope this helps you abit.