It's easier to explain what I'm trying to do with pictures.
I have this structure:
And I want to achieve this:
such that those two new edges are equidistant from the first, and the distance is set precisely (using the keyboard).
I've been doing this using two
Edge Loop (Ctrl-R), but I need to do the math by hand, since it always counts the cut distance from the left. Is there a easier way?