What you guys refer to is called slippage. You can look it up at Investopedia. Before setting stop loss you thus shouldn't look only to under which support you want to sell. Best thing is to try and get a view on what the strength is under the support so you can adjust accordingly.

The 1% is a general good idea but can vary greatly from stock/asset to another.

Should it happen that slippage causes you to be "stuck" in a trade there should be no need to worry. I try to trade with the tokens/coins I believe have a strong project/foundation. This makes sure I never panic sell, but be in the trade a bit longer than planned.