How do I determine figured b in a chorale?

Musicians, I need your help again =]. I CANNOT figure out how to work backwards with figured b. If I'm given the FB in an exercise with the key, I can write the chorale just fine. If I'm given a 4 part chorale I can determine the roman numeral ysis but I can't figure out the figured b. What confuses me is the doubled tone. We're covering the cadential 6-4 and 7th chords and their inversions. I really want to know how to determine the FB before it gets too advanced and out of hand for me. Please help!