How would I make perfect diagonal edges in a rectangle In the pic attached I'm trying to make a perfect diagonal edge across the face as I've so beautifully illustrated in MS paint. I've tried subdividing and moving the vertices along the edge so that they look close enough, but 1) they aren't the exact distance and 2) I have to create the vertices on the opposite diagonal side which is a pain. 

And I have tried using the ruler/protractor tool but that's been fairly useless so far because 1) it disappears when I go back into edit mode so I can't place my vertices exactly where it was, and 2) you can't enter exact values in it, so when you get down to a measurement like 3.4000005cm you'll never be able to match it on the other side (if that makes sense).

Any suggestions to go bout this? Theres probably some stupid easy solution to it but I've been at this for HOURS and still can't figure it out.
One way to get diagonal based lines like that is to use Offset Edge Slide which will add loopcuts on both sides of the present edge. However one edge is needed, so select opposite vertices and press J to join them with the edge and split the face.

Then select the edge, open Toolshelf (or press Ctrl+Shift+R) and drag the mouse for the effect.



Regarding the edge you create for this tool to work.
You can dissolve it afterwards but note that it creates quite horrible Ngon in the middle which is better to avoid. It all depends on the purpose of the model but it's best to stay away from this geometry to avoid any problems later on.