A tip from Alun Potter
   
   
If you want to offset a line equal distances from a central line and you only know the overall distance type x/2 as the offset distance.

For example, if you have a line which needs to be at the midpoint of an overall offset of 762mm, use the following command: Offset - 762/2 - The line can now be offset 381mm either side.
A great command for non-whole numbers or if you're not great at maths!

I personally use this one when I'm putting windows or doors on a floor plan and I want them to be central to the room, hence the midpoint.