You can look up the detuned tension in the tables at the D'addario website: http://www.daddario.com/DAstringtens...f-446438efa151
That page explains how to use the tables and there's a link near the top of the page
for downloading the full table.
All their strings are listed with tensions for various tunings.
The calculation is relatively simple, though. First find the frequency ratio between the standard tuning tension,
and the detuned tension. The frequency ratio is approx 1.059 for each half step. So for a whole step, the ratio
is 1.059 x 1.059 which equals 1.121
The tension is related to the frequency squared; so square that value to get the tension ratio - 1.121 x 1.121 equals 1.257
Now just divide each standard tension by 1.257