Yep, it will work using the relation
sd0 = rsd1 * 9/16
for the two perpendicular sides of the rectangle, where sd1 is changed to a reference dim (rsd1) before creating the relation. I can then update the (strong or locked) diagonal dimension sd2, and the whole rectangle follows.
However, it does seem a little flaky in regenerating every time (might require multiple regens before I'm certain that it's all updated, and doesn't handle large changes in the diagonal length) so I'd agree that the angle method is likely to be more robust.
I have to agree with Frank: this is a basic case of "necessary and sufficient" constraint. A rectangle is defined completely by two dimensions and a (set of) perpendicularity constraint(s); any more dimensions create overconstraint.
There's no difference between 'strong' and 'locked' for constraint purposes; 'locked' is mostly useful for controlling what is flexible when you drag a sketch (occasionally useful for significantly changing the shape without deleting everything and losing references).
Oh, and: AutoCAD has something better than Query Select? Where do I enable this?
