For feathers in a border: first I measure the borders both width and length. I mark a "miter" line in every corner (diagonal line) then I mark the half way line in both the width and the length. I take into consideration binding and squaring up and adjust my width center line accordingly. I then take my length wise measurement and see what it divides into evenly and still look nice. Then I grab my circle ruler that is closest to that measurement and mark the undulating spine of the feather going back and forth across that center line I drew previously. I do all this before I load the quilt on the rack. The spine is all I mark for feathers. The rest is freehand. Really the human eye is a remarkable thing. So even if every plume of the feather is different the human eye "sees" them as being uniform when viewing as a whole. I use the "hump and bump" method for quilting feathers so every other plume is backtracked across the top. I am human so there are loads of backtracks that aren't exactly on the previous line of stitching, but again that marvelous feat of intelligent design (my belief) that is the human eye corrects for that and unless you are scrutinizing every plume you don't notice those backtracks where I am a bit off. I have found the less I stress about them the better I get at them.
I like my feathered corners to wrap around the quilt so I find it is easiest for me to "stop" the feather at the mid way point on every border in order to wrap them around each corner the same way. I usually have the feather swirl back on itself at the midpoint.
For the puff top feather, swirl thing in the block, I used the piecing itself as registration dots. So at each stage of the "swoosh" I was looking ahead at the point I wanted to hit with my eye. Again those marvelous human eyes, where you are looking is where you end up going. It is the same when driving down a road. You aren't looking at the road right in front of your car are you? No of course not you are looking way ahead and miraculously you are steering the car where you want to be and managing to stay in your lane. It is the same with aiming a gun, archery, shooting pool and quilting. Look ahead to your target and you are sure to hit your target or at least darn close to it. So no I did not mark those. But if I am doing a feather in a block, I will mark the spine.
Believe me each one of those swooshes are slightly different but because of the way our eyes work we see them as being the same.