I just caught on you're using a stepper, not a servo.
Close enough is good enough. I am not tracking a UAV or anything :D
That's what I figured. This is my suggestion off the cuff.
As you have a 10'-15' accuracy, that will be the basis for your threshold. You might want to consider half that. If so, that's a 5'-7' window. So find the strongest signal with that threshold and stop. Again, since speed isn't a requirement, I'd probably do a full rotation for a sweep, recording signal to stepper position. Smooth the signal a little and the look for edges - not peaks. Well, look for edge, peak, edge, so you know when you found the two corresponding edges. The two edges will be your focus. This will become complicated if there are multiple sources within your threshold and IMOHO, becomes the most difficult part of the solution. If that is the case, we're wondering into signal processing territory, unless you just want to blindly look for peaks and make assumptions.
Now seek back to your first edge and slowly start sweeping back toward your other edge. Slow as signal rises. Again, record signal and position. Smooth signal. Based on your gear ratios, you should know the degree of a step. Calculate the degrees from edge to peak. Start walking back toward the peak until you find peak signal and you're within your landing zone of 5'-7's. Stop. You're done. That should be in the "good enough" ball park.
Now repeat for each additional edge, peak, edge detected in your original sweep.
Obviously it can be refined some more over time but that should get you into the close enough range. The important part is to know the steps per degree and to understand what type of signal you're looking for and why type of compromises you're willing to find in your good enough solution. Remember, a really strong signal could be two AP's inline from your POV or two AP adjacent to each other - so on and so on . Automatically discerning that can become a DSP problem.
Something else to keep in mind, I have no idea what type of precision you'll get out of your signal source. You may need to further tweak some based on the speed of delta and precision of signal measurement.