I think it's not supposed to be arbitrarily executing trades immediately upon being enabled. Fix that bug & no threshold fields are necessary.
It would be the simplest option: wait for a cross to happen and buy/sell depending on the cross and the availability of BTC/FIAT.
That's what I had originally assumed it would do, but it surprised me. Worse than that, it kept sending me sell triggered messages after I disabled it. There was no more BTC left at the time so I don't know if it would've actually sold while disabled or if it was just email ignoring the disabled command.
Undortunately I closed Chrome in order to stop its buggy behavior, and thus lost the pertinent logs for debugging. So once I get more BTC, I'll try to recreate it so I can send logs so it can be fixed. :-)