No, it's not subjective at all.
It's actually entirely objective, in terms of the amount of tension you need to hold in the muscles connected to your shoulders, elbows, and wrists. You're looking to maintain a neutral position in all of those, to minimize tension there. There's nothing subjective whatsoever about that, tension is very observable.
Second, I would very much push back on key wells for the general population. Adjustable tilt forwards on a flat keyboard achieves everything you need for neutral wrist position and relaxed fingers, so that you're always typing slightly "downwards". Key wells try to accomplish a weird goal of holding your arms/hands in a fixed position and typing with just your fingers, instead of typing with your whole arm floating in an integrated shoulder-to-fingertip movement. If you have some kind of arm movement problem/disability then concave wells might be the best solution, but for the general population the wells are more awkward to use in the upper rows of keys, increasing the forearm tension which controls the fingers. (As a general rule, tension is usually minimized overall by distributing work across as many muscles as possible that involve longer muscle trains, rather than localizing work in shorter areas. That, for example, is why people say to lift with your back, not with your arms.)