What is ur rule for “add to winner”

I currently risk 1% per trade and have 2:1 risk/reward ratio. Read many books that strongly suggest you to add to winner but by how and by how much? I once try to add share only if I passed my first 2:1 TP, I sell 50% of my share and 50% for runner, then wait for potential new entry, but I find I missed a lot of perfect setup before my 2:1 TP is met.