Here are two ways to do it without using a loop. One uses In Range & Coerce to get a boolean array of peaks within the desired range. The other uses Threshold 1D Array on a reversed array.
Note: If you choose one of these methods, you will need to extract the same subset of the Locations array.
If you were to fit the data to an exponential as I mentioned earlier, then you would need both amplitudes and locations. For your ratio method you do not need the locations.
