Scaling out of positions: best practices?

Hey everyone, still relatively new to CFD trading here, specifically with $SPX500 and $GER30. I've been experimenting with scaling out of winning positions, taking partial profits at key resistance levels. My thinking is to lock in some gains while still giving the remainder of the trade room to run. However, I often find myself taking too much off too early, missing out on bigger moves, or conversely, leaving too much on and seeing the profit erode. For those of you who have been doing this longer, what are your general principles or considerations when it comes to scaling out? Are there specific percentages you aim for, or is it more about price action at certain levels? Any insights would be great.