How is threshold calculated?

It varies, but given the version you cited.

If you measurement for current is say 650, then 651, then 649, then 700 mA across 4 frequencies, the running average up to the 700 would be 650 mA. 700 would be a change of 50 mA.

700 / 650 * 100 = 107.69, which I believe is reported as a hit of 7.69 which is way over any threshold one may specify (big hit). Increase of 7.69%

A threshold value of .2 would eliminate all the data that falls below this, meaning it would have had to increase by .2 % (given the version you cited).

I'm not 100% sure this is it, would have to run some numbers to verify, but not in a position to do so as my gear is back at home and I'm in Alt. GA

If you were using peak, instead of running average, then the raw data is used.

650, 650.18, 650.5 for 3 measurements. 2nd frequency would not make the cut having a change of only .18 (below .2), but the 3rd frequency would having a change of .32.

For more details, please check the link:
https://www.spooky2.com/forums/viewtopic.php?f=5&t=9645