Rectangle mix inputs and selections?

[Spiderguy]

Sorry about the vague title... I was wondering about mixing input types when inputting the size of the rectangle. I often know one dimension of a rectangle but don't know the other, for instance, when drawing a 1/2" wide rectangle, I'll select the origin point then i type in 1/2", type in an arbitrary number to get the rectangle on the screen, then grab the grip and drag it to perpendicular to a line that is at the length of what I need... Is there a way to type the distance for either x or y and make a selection for the other?