Jeff, I don't pretend to have any expertise on this but for what it's worth I can pass on something I was told by a designer. His view was that for light airs, sail area/wetted surface area was the factor to consider. He recommended a minimum value of 2.5. 

Calculating wetted surface is tricky, but there are a bunch of calculators online that give a good-enough guess if you're interested.

Jeff ,
here is the method for calculating SA/displacement:

 http://www.junkrigassociation.org/Resources/Documents/
Arne%20Kverneland%27s%20files/How%20to%20calculate
%20Sail%20Area%20to%20disp.%20ratio,%
20ver.%2020100923.pdf

It isn’t just to divide the SA with displacement, as you do, although I have started to use that method as well. Then the answer comes in square meters per (metric) ton.

By using the formula in the link above, your boat’s “proper” SA/disp should be 23.6.

That is a bit higher than the..

“Direct SA/disp” = 25m²/1.116ton= 22.40m²/ton

If you are to sail mostly in coastal and half-protected waters, I still would recommend going for those 25m². However, if you are to sail in a generally windy area, offshore, I guess 20m² would be enough.

As for calling the Johanna type sailplans for “state of the art”, I am not sure I am with you there. There has been done more recent developments of the JR, which may well be faster than the rigs I design. The Johanna type sailplan is just a well-tested sail which is easy to make, and which performs and handles well. That is good enough for me.

Good luck with your project.

 Arne