Feather Fixer is essentially a gimmick, IMHO...
It basically has 2% more protein than layer at 18% verses 16%. I would go all the way and get a flock raiser (20%) or un medicated starter (22%) for at least the first bag or until the issue is corrected. As long as you provide OS on the side, it will work for everybody in your flock.
Maybe watch a video of peepers being applied. I've never done it but it always good to see examples, I would think...
I wouldn't call it a 'gimmick'. It isn't just about the amount of protein. Feather Fixer has chelated trace minerals to support feather regrowth and sulphur which is, as far as I know, one of the few effective treatments against knemidokoptes laevis var. gallinae, commonly called the depluming mite. It isn't a gimmick; it's a different formula.