Place it right below the most apparent psychological level. If the stock has bounced at 79.5 three different times, put at about 79.45 or 79.4. Or if there's no apparent support level, people tend to make decisions with round numbers in mind (79.0 is a good round number, so the stop could be placed right below at 79.8).
The tighter the timeframe, the tighter your stoploss (must keep a greater control of risk). Risking a nickel or dime would be sufficient for a daytrader, but obviously, on the larger timeframes, you're going to have to allow wider stops in order for the trade to develope.