I would do this manually, not in any auto mode. Why - you (i) don't want to have a standing (or even auto induced) order out there at 5% below the market as you risk buying into a true black swan event, probably terrorist related (use your worst imagination) or some "act of God" that knocks out electricity or communications. Even if you have a stop loss order attached to the buy, there is a decent chance it would not get filled before the markets closed (worst case scenario, hopefully very unlikely but .... there are lots of fervent humans running around out there now.)
The terrorist event in particular, is something that would cause the market to quickly go limit down and then close for days and days. When the markets reopened, there would be no buyers initially except maybe for oil, gas and currencies.
And even in normal selling, rather than pick an arbitrary percentage like 5%, I would use support and resistance zones to make the buying decisions. GL.