How to set this ruler? Two clumsy methods:

First, sell in batches. Don't always think about selling everything at the highest point. You can sell 30% when it rises 20%, then sell another 30% when it rises 40%, and let the remaining profit run. Even if it falls back later, you've already pocketed 60% profit.
Second, use a trailing stop-loss. Don't watch the price, watch the stop-loss line you've set. For example, if you make a 20% profit, move the stop-loss line 5% above the cost price. If the price continues to rise, move it up. When the price drops below your stop-loss line one day, sell automatically. This way, you'll never sell prematurely because the profit is always following you.
Selling prematurely isn't scary; what's scary is never learning a lesson. Starting today, set a profit-taking ruler for yourself. When it hits, sell without regret. You're not selling opportunities; you're taking the rewards you deserve.