Tall Paul had one of the simplest and most obvious rules of thumb. Look at the top view and do the CG as if it's only one wing with the area being only what you can see. Don't worry about the extra area hidden under the top wing. Only what is visible in a top view is the outline of the equivalent single wing. Assuming it's not an overly small tail it'll work as well as anything.

Besides, the FB2 had same sized wings with no stagger so the CG is the same as if it was a single wing.