The end bends are about 6" long so moving the mounts about 2" forward would stiffen a bar about 2/6ths or about 33%?

 

If you check with the manufacturer of the bar you like, they can tell you the percentage stiffer than stock for each setting.

 

Claud is correct, the manufacturer should list the percentages of change.
Don't know what brand you have but I've got an h-sport 19mm set in the middle setting and it's perfect for my type of driving (daily use and a little spirited road running, no track). Really takes the understeer out.

 

It's a little bit more complicated than that, but still pretty simple.
Here's the standard formula:
http://www.klaindustries.net/technical/drplink.htm


Plugging in 3/4" (19mm) for Diameter and 35" for the mid-Bar length (a wild guess - please
measure yours and re-do the calculations for better accuracy) gives 270 lb/in for 6"
perpendicular ends (A = C), and 626 lb/in for 4" ends (a 132% increase)!