If you want to implement zooming, I would suggest using the ScaleTransform method of the Graphics Class to do the scaling and avoid all the math coordinate calculations and manipulation for your drawing.
If you draw the image and then all the lines in whatever coordinate system you choose, you can scale the drawing up or down or even rotated without having to modify the coordinates of your drawing at all.
This post includes a couple of links that references a number of attachments with various examples of drawing situations. In particular, the "VB_Drawing_Demo" and the "PaperPuppetExample" both make use of the ScaleTransform to increase or decrease the size of the drawing without having to change any of the core drawing routines at all.
"Anyone can do any amount of work, provided it isn't the work he is supposed to be doing at that moment" Robert Benchley, 1930