Before try to purchase any coin include bitcoin, make sure that they already reach quite low value so you can wait until you've reach in profit.
When you say a low value, you're being vague as what is a low value this moment can become a high value next time you check. To be precise, we need to employ couple of trading tools that enable us pinpoint when market is oversold, which implies price is then low or if overbought, price can be said to be high.
But if things got worse, for me if loss more than 10% , I will sell it right away by using sell order for auto sell on spesific value because in cryptocurrency, everything can happen !
10% stop loss for Bitcoin is more or less same as indulging in panic selloff and should you continue at that, won't be long your account would likely run dry.