Studying under Uncle Brad and Uncle Accumulator, I've now had a couple of shots at situations like yours. I was working to polish a resprayed area. The PC with yellow pad (the big pad that sticks out beyond the backing plate) worked well for me, but the first time I tried it, I got down too far and worked back down to the original paint. Bore down too hard, apparently because maybe the pad generates more "energy" when you're just using an edge of it. Like Accumulator said, the pad margin gives you some "forgiveness factor." This time (like, in the next couple of hours), I'm going to go gentler when using the edge of the pad.
Also, like Accumulator suggested, cover the car parts that you're not polishing (I use old sheets) so you don't sling product to Kingdom Come. And tape-off the rubber/metal trim that you aren't polishing.
As to your base question, try to keep the PC "in plane" with the main part you're polishing, so you don't go too deep on the vulnerable edges.