Your selling price should be your own decision, I know that all of us want to sell at the ATH to maximize the profit but there is really no way to know that. If you are contented with your profit then you can sell that altcoin, don't regret even if it goes higher. Learn to control your emotion when you are getting greedy, just think of what will happen if you didn't sell and that altcoin starts dumping. There is no right time to sell, it is just our greediness that consumes us when the market is rallying.