Your stop depends on many things, first your time frame that you trade, for example if you are trading using a day chart, then you can use daily pivot points as SUP and RES and also not forgetting the weekly pivots, just as TYGMA mentioned. If you are trading on a 4H chart your stops will probably be narrower, the lower the time frame the narrower the stop unless you are entering on short time frame charts with the purpose of exiting on longer term time frame charts.
Assuming you trade one time frame (Day, 4H aor 30M) you can set your stop loss in line with the volatility of the price (which also changes in time).
Example, you want to scalp the NSDQ100 on a 30 min chart today, so you check the updated volatility of NSDAQ100 in different time frame and you focus on the latest 30 min volatility (which is less reliable than the 4H volatility but more reliable than the 5 min volatility) and lets say the number is 0.16% so this means you can set your stop loss at 16% (0.16 X 100 for 1:100 leverage or 0.16 X your leverage). it would be wise of course to enter at an extreem low/high or at a crcual pivot point and then place a stop loss in line with the volatility at that level.
In all cases make sure your stop is based on Technical Analysis, (below a crucial Sup or above a crucial Res), how far the stop depends on your time frame
Since longer term charts are more reliable than short term charts you can use a Day or Week chart and check the volatility (which will be bigger here than short term charts - eg: 30M…)
Another way to check volatility is using bolinger bands or two Moving averages, one for the highs for the last 20 bars (for example) and one for the lows for the last 20 bars.
Put in mind volatility changes hence whenever you make a trade dont use the last % stop in line with the last volatility but check latest updated volatility and use that BELOW the support or ABOVE resistance. this way you combine technical analysis with price volatility and it s harder for your stop to get hit. Good luck!