The brix reading is incorrect- sort of. When you take a reading, the brix reading is telling you the amount of potential alcohol remaining. So technically it's correct, but it's useless information.
What you need to do is take the original brix reading, and subtract the current brix reading for the approximate ABV. It's easier to do that in the SG scale, so you'd convert the brix reading to the SG reading first.
The formula for the ABV is (OG-FG) x131 = approximate ABV