Got it working with a diagram, making heavy use of the displayAreaWidth and displayAreaHeight variables to replace screen1 width and height. Also had to adjust the conditions for when zoomed as I guess the calculations are not return exact values.
WiringDiagram.aia (497.8 KB)
There is a gotcha, in that there is a bug in the canvas background image display - it may go fuzzy(your image). MIT are aware and are looking at fix.
For more on this see here (there is a sort of solution):