Stand at the net, in the middle of it. Hold the phone up and turn round on the spot until the circle fills in. Either way round, start wherever you're facing.
The middle is where it has to be taken from, and it is worth ten seconds of walking: the same trees stand forty degrees higher from one baseline than the other, so a reading taken at the wrong end is a reading of a different court.
Any daylight, any weather. You're measuring the skyline, not the shade, so midday sun, cloud and drizzle all work — the camera only has to see sky. After dark it can't.
Once, and it holds for good. The skyline does not change, but the sun does — so the app recalculates the shade on your court every day of the year off this one reading.
Takes about a minute.
A reading taken off a kitchen wall is a reading of a kitchen wall. The app would file it against the court and then be confidently wrong about the shade there for the rest of the year, which is worse than having no reading.
Shade is different on every court, so the reading is filed against the one you're standing on.
The reading is safe. Which venue, and which court — then it bakes the year.
It has to be one numbered court. Two courts at the same club can be opposite halves of the day: at Killara, court 1 goes dark at four and court 2 is dark until ten.
Every reading saved as you took it, on this phone, and nowhere else. Nothing about your court leaves it.