I’d appreciate if it was possible to set the scale of the preview as a number. That would be more important than setting the scale in the drawing area. Dragging the vertical size is nice, quick and intuitive but generally, I’d like to see the preview at exactly 1:1 size, which is impossible to find.
As a simple solution, it could be implemented as:
(1) Limiting the preview size to UPM pixels (i.e. 1:1 scale). I don’t think one would need an “enlargement” compared to what you draw. So you could simply drag it all the way up to get 1:1.
or
(2) Having a litle “notch” or “snap” at the UPM so that it can be easily found while dragging the vertical size. Similar to the notch in your stereo’s balance knob.