Hmm. Almost all piano playing hand technique is about relaxing. Focus on that. ====snip====
Try this (not at the keyboard). Relax. Where do you fingers rest when you lift your forearm?
In my case I lift my forefinger slightly in both hands and lift + straighten the pinky slightly. It's not a problem and is just the physical conformation I happen to have.
Don't stress about this too much.
AJB - if I follow your procedure, it is designed to help a person find what I, and Peter Feuchtwanger (as I recall), would call "neutral position", that is, the position of the hand when relaxed.
If I've got that right, may I suggest yet another method to find neutral position that I have used with my students?
If the student is physically able to stand, have them stand, arms relaxed, hanging down straight. If they must remain seated, have them sit straight as possible with the bench or chair turned so that the arms can hang down freely without contacting the seating surface.
Once in that position, I ask them to breathe and with each breath focus on consciously letting go of the arms so they hang completely freely - the arms should feel almost like dead weight once a person has relaxed enough.
It is at that point, in every case that I have observed, that the hands will assume what is "neutral position" for that individual. The fingers will curve just so, the thumbs will bend back at the first joint (or not), the thumb will be a certain distance from the rest of the hand, the other four fingers will usually hang together touching.
Then I have them LOOK at their hand - usually in a mirror - to see what neutral looks like.
Finally, following what I learned when Peter Feuchtwanger did it with me, I lift the seated student's arm onto the keyboard with the thumb hanging off, and the other 4 fingers on any white key.
"Quick Release" is about laying the foundation to play that way, but, in most cases with even more flattened fingers, confining the curve to the first joint (the one furthest from the palm).
It's an exercise that looks simple to do, and eventually becomes one that is, but, in almost every case, it is not that simple to learn to develop the awareness to be able to isolate the particular muscles involved in pressing a key down.