You can do iteratively, by hand, at least with 0.91:
Draw your shape. See my beautiful walrus:
In the extensions-menue, there is a point - in German it is "Erweiterungen/Pfad visualisieren/Pfad ausmessen" In English it should be similar to 'Extensions/visualize path/measure path'. You mark the path in question, select 'area' from the drop down, and 'perform/commit/enter' (anwenden).
Now the size is printed as in the upper left corner. Now we calculate that value to 20% or whateve we need, which gives me round about 14.000.
Create a duplicate of the path, and remove the upper nodes, pull the lower nodes down, measure, repeat.
It might be hard for a figure with many interceptions and a lot of work, depending on how exactly it has to fit and how complex the figures are, but for an error below 1%, I guess it is doable.
Bonus quesiton: Why is the size negative? I have no idea. :) 5 positive vectors, 3 negative ones, multiplied?