The D'Addario website has the formula. The trick is you need to know one variable which is specific to the actual string you're using, related to ratio of core to winding, type of metal, etc. D'Addario provides that number for all their strings. Here's someone else's applet that does the calculations:
On Sun, Dec 14, 2008 at 8:03 PM, Toby G <carpet8@mac.com> wrote:
Well if you adjust the tension of the 38 gauge strings so it's no higher(or not much lower) than what would regularly be there, it should be ok. One of the physics guys on the list might be able to calculate that for you.