There are some rule of thumb guidelines which are --crude --but get you into the ballpark.
The problem is that these guidelines all shift , if you look at a sport flyer type -vs a good aerobat vs a "scale type flyer.
The last set of guidelines is for the old "warbird" set.
The standard gag was that warbirds flew scalelike
The realworld was that most of them were simply overweight .
I remember looking wide eyed at "how to" columns , which explaine how to trowel on automotive primer onto plywood , in layers , etc., to make panel lines.
Also watching footage of the great builders whose scale planes simply shed their retracts on "landing".
Some very rough parameters for powered aerobatics:
electric aerobats under 300 squares -- 5-8 ozs per sq ft--10-16 ozs all up
little 40 size areobats -if they used that 1 lb per 100 squares , would be at almost 23 ozs per sq ft.
That is awfully high.
but go to a 1200 sq in 12 lb model -23 ozs per sq ftis a good loading (1 lb to 100 squares)
Now the ratio shifts the otherway
2000 squares can easily fly 28 lbs. (32 oz per sq ft.)
many older kits were simply grossly over weight.
The CAP 20 kits once sold by Great Planes earned a reputation of "snappin Caps".
Caps are NOT snappers - just heavy the heavy ones.
there ain't no magic number - but the rapid change in RN at these small sizes,really shows up.