The FXScript Reference

renderRes value of 1 = 100% of sequence frame size
renderRes value of 2 = 50% of sequence frame size
renderRes value of 3 = 33% of sequence frame size
renderRes value of 4 = 25% of sequence frame size

I can't get renderRes to report anything other than 1.

My expectation for this and what would be useful is a representation of the currently visible preview's zoom amount. For example the Canvas window at 25% would report renderRes as 0.25. That value could then be multiplied by whatever values were used to scale them appropriately.

There are examples of this not working in Apple's own FCP filters. Apply the "Widescreen" filter (in Video Filters -> Matte). Set a visible border with a discernable width. Now zoom the window in and out. Before rendering, the border width will remain fixed at all zoom amounts below 100%. The border-width value in that filter is specified as borderwidth/renderres.

Well, now it's working in a generator and a filter, but not in the same code switched to a transition.

Referencing Martin Baker's comment above, when RenderRes does return a value, the scaling value can be found by dividing RenderRes into 1:

zoom = 1/RenderRes;