What this sheet works out
Where the mass goes
Group fractions against the usual range
How these are worked out — the equations behind this sheet, their sources, and what each one assumes.
The budget, line by line
Where people go wrong
The errors this method actually invites, and what it does about each. Every one of them is already a warning somewhere — in a note that appears when your own figures trip it, in the scope statement, or in the equations behind the page. Collected here so you can read them before you need them.
Treating the areal densities as measurements. They are compiled from build practice, and the spread between two builders working to the same description is wider than the gap between two rows of that table. This is the softest data on the site. If you have ever weighed a wing, that figure beats all of them and the sheet will take it.
Adding the contingency to the payload. It is excluded on purpose. A payload is usually a known object that can be put on a scale; the contingency is for the glue, filler, paint, tape, extensions and nose weight that accumulate on everything else.
Expecting the fuselage ratio to be derived. There is no parametric fuselage formula here, deliberately. A model fuselage runs from a carbon boom to a scale cowl with a retract bay and nothing parametric spans that honestly. It is a ratio you set and can argue with.
Forgetting what the areal figure already covers. Spar, ribs or core, covering, hinges and horns — finished structure. Servos are not in it and are budgeted separately, so counting them twice is the easy error here.