Isn't 3% left in your account is too small? There is nothing much after you sold 97% of your bitcoin.
What I follow on my altcoins is I sell 10% every 2x of the amount. So if I bought at 20, at 40 I will sell 10%. Then another 10% on 80, 160, 320, 640 and so on. But I only follow this if I believe that the altcoin I am holding have great potential.
You're assuming here that the person only sells the other 97% and never buys again. He may buy and sell and buy and sell and buy and sell multiple times, but never touches the original 3% and any additional 3%.