I do 21° because that’s what I figured out for my homemade jig which involves a Lansky clamp, a small steel block, a steel rod, and a microphone stand. Very Rube Goldberg. I lack the ability (or possibly the patience to acquire the skill) to hold an angle that is necessary for good hand sharpening, but I have finally worked out a deal that I *can* do ... with a 1x30 HF belt grinder and some fine-grit belts (and a leather one that actually seems to make a big difference).
Now if I had three grinders so I wasn’t changing belts all the time... But seriously, I am happy because now I can actually have sharp knives all the time. Only took me sixty-some years to figure it out, too. Imagine what I’ll come up with in another sixty.