EI is an estimative way of dosing, not a precision way. So, unless the sump is comparable in size to the tank it probably makes no real difference whether you add its volume in or not. And, It is certainly true that the thick substrates we use displace a lot of water, further reducing the process to an estimate. If I had that big tank and sump I would ignore the sump volume for two reasons, first that the actual quantity of water in the tank is considerably less than its internal volume, and second, the plant mass is at a minimum when you first set up the tank. After a few weeks I might decide that my plants were growing slower than I expected, and then I would increase the dosing. But, it isn't an important thing to calculate.