First, it assumes that you will be using a good quality foam such as blue foam or Spyder foam, so as to reduce buckling failures of the skin in compression. Second, it assumes that you are using material for the full chord, no triangular darts of material. Third, it assumes that you will be careful in your epoxy application, no extremes, just that you are not pouring on the epoxy. Fourth, it assumes that all fiberglass is put on in equal amounts, top and bottom, but allows different lay-up schedules for the top and bottom for carbon fiber. What is not included in the weights, is ribs for joiner load distribution, tubes for joiners, etc.
All input data is in yellow.
The output span, wing area, and wing volume are for a single half of the wing. Using the spreadsheet The usual way to run the spreadsheet is to modify the wing planform to match your proposed new creation. It has to have three panels or less. If you are using a two panel wing, break the inner panel into two panels, and if one panel, break it into three panels. Make sure that you modify the wing planform shape factor if you are using something other than the typical efficient 3 taper wing that people have been using lately.