As soon as the sides of a box lean in or out, one saw setting is no longer enough. The joint between two leaning pieces is a cut through a plane that is tilted in two directions at once, so it takes a miter and a bevel together. This page derives both rather than quoting a chart.
The two formulas
Call the tilt from vertical T, and call the flat half-corner angle A, which is 180 divided by the number of sides.
The miter — the swing of the saw table — is the arctangent of cos T times tan A. The bevel — the tilt of the blade — is the arcsine of sin T times cos A.
Check it against the case you already know. If T is zero, the sides are vertical: cos 0 is 1, so the miter is just A, and sin 0 is 0, so the bevel is nothing. That is the flat miter, which is what it should be.
Crown molding is the same problem in disguise
Crown molding sits at an angle between the wall and the ceiling, and that angle is quoted as a spring angle measured off the wall. The two common ones are 38 and 45 degrees.
A spring angle is the complement of the tilt from vertical, so a 38 degree crown leans 52 degrees from straight up. Feed that in and the formulas give 31.6 degrees of miter and 33.9 of bevel — which is exactly what the published crown charts say.
A 45 degree crown gives 35.26 and 30. Also exactly the chart values. The advantage of the formula over the chart is that it handles the corner in your house that is 87 degrees rather than 90, and the chart does not.
Which number goes where
The miter is the horizontal swing of the saw — the setting you change by rotating the table. The bevel is the tilt of the blade away from vertical.
Saws differ in which direction they call positive and whether they bevel left, right or both. The numbers here are magnitudes; matching them to your particular saw's scale is a matter of one test cut.
If your saw bevels only one way, half the corners get cut with the workpiece flipped. Mark every piece for which face is up and which end is which before cutting anything, because a compound cut on the wrong face is scrap.
Hoppers, planters and speaker boxes
Anything with sloping sides that closes on itself is this calculation. A four-sided planter that tapers, a six-sided hopper, an octagonal lamp shade, a canted tool tray.
The tilt is easiest to get from the drawing rather than measured on the work. If you know the top and bottom dimensions and the height, the tilt is the arctangent of half the difference in width over the height.
Do not try to work it out by trial on the saw. Compound angles are not intuitive and the errors do not behave the way flat miter errors do.
Why the joint faces get shorter
One consequence that surprises people is that on a canted box, the pieces are not as tall as the box. The side panel is longer than the vertical height because it leans, and its ends are cut at an angle so its long point and short point differ.
Lay out the side panel from the drawing on the tilted plane, not from the elevation. Cutting a canted box from pieces sized to the vertical height gives a box that is too short and whose top edges do not meet.