Now, there's an article that I totally should have put together a long time ago... On the to-do list it goes!
For the short version - The easy method is just to mark either side of the base with a strip of paint or marker pen.
Or you can get a little more fancy, like this:
...which was done by marking either side and then carefully filing the grooves in with a needle file, to more or less match the width of the arc grooves in the standard MEdge
To find the middle line, put the base on some graph paper or a gridded cutting mat, line it up with the lines and use that as a guide to find halfway.
Or, use a ruler to find the widest point on the base, and make a mark either side.
Hope that helps!