I’m a GP, here’s my opinion

Can’t have eaten/drank anything for the last half hour

• in principle could alter your BP but I wouldn’t worry too much unless it’s quite a large meal

Feet flat on the floor

• yes, this is important

Lying down but sitting up

• for some purposes docs want lying/sitting/standing but for home measurements do them sitting

Back against the chair

• yes

Don’t cross your legs/ankles

• yes, feet flat on the floor

Only use your left arm

• myth, if there is a significant difference between your left and right arms there is something funky going on with your subclavian arteries

Hand facing upward/downward

• not super important

Keep your arm down/raised

• keep your arm relaxed, ideally resting on a table or desk at close to 90deg or hanging straight down

Most important is be relaxed, sit still, don’t move your arm, if you get a high reading calm yourself and take it once more then leave it.

When I’m taking a BP in clinic the most important thing I do most of the time is distract the patient from the machine with some patter as for most people the biggest confounding factor is stressing about what the reading will be, I don’t correct posture etc unless they are substantially moving their arm around.

I have taken my own BP manually, it ain’t easy

You’re probably talking about the Chinese social credit score, not a replacement for currency but is up and working.

Modern guns a extremely precisely engineered devices that are incredibly easy to use, for better or worse. I know modern sporting bows are also but it’s no contest in my opinion.

I’ve shot both, bows as a complete amateur and relatively competent with a rifle. There is no question that a modern gun is way easier to pick up as an amateur and hit what you want to hit and I cannot possibly believe there are anything other than extremely niche uses where a bow is superior.