Not really. Think angles…
If its square, the rotation is obviously 45 degrees (the two radian figures above are 45 degrees and 135 degrees)
If its rectangular, then the angle gets closer to 90 as the width increases.

If sin(x) = height/width (opposite over adjacent)
then I guess I need asin (height/width) rather than asin (hypotenuse)

exactly what are you trying to rotate then?
As Lanque mentioned… an oval can be encased in a rectangle (as a matter of fact that is how Xojo draws a “normal” oval)
are you trying to rotate the “object”? or each point along the path of the object

You need trigonometry. The tangent of an angle gives you the rise divided by the run (vertical divided by horizontal). Since you know the height and width, you need the inverse tangent (aka atan) function to get the angle.