Update...
I've done some calculations today...
From the blueprints of the planes and the help of some tool here's what I got.
Sadly some weren't really detailed (he 111 was a image of 1400 x 1000) so there could be some marginal errors.
squared meters squared feet
Plane front side belly front side belly
bf109g6 7,9 9,9 23 85,03 106,56 247,56
spit mk1 8 11,3 28 86,11 121,63 301,38
He 111 17,2 36,5 124 185,13 392,88 1334,72
Our engineer has drawn a mathematical function for the graph:
y = 1,833 + 0,0167x (100<x<190)
y = 0,011x + 2,9 (200<x<300)
y = 0,009x + 3,5 (400<x<500)
y = 0,007x + 4,6 (600<x<700)
y = 0,007x + 4,5 (800<x<900)
y = 0,006x + 5,3(1000<x<1100)
y = 0,005x + 5,8(1200<x<1300)
Here is the new graph with meters and the new planes:
FV = front view
SV = side view
BV = belly view