Several exchanges allow for that type of setting, but a stop loss has also proven to be dangerous, unless you are able to monitor your position full time. I basically gave up using them, because the price would dip while I was at work, I'd get home, log on only to find my trigger was met and I am back in cash. The market however recovered, and the BTC was listed $10 more than when I went to work in the morning. Admittedly, the market is more stable now, than in 2013, but I still don't use auto trades anymore. A 24hr market is tough as well, since when the USA trading tapers, the China/Japan trading kicks in... when the hell can a guy get some rest.