Just measure the width and height of the plate, mark the position half the width minus the hole size to the left and right of the hole. Do the same for the height. Now you have the exact positions the edges of the plate should be positioned. Didn't you guys have math lessons in high school?
The problem with that method isn't the
math, it is that it's very easy to get a few mm off on the
markings when the center hole is already drilled.
When you use the mounting plate as a template, you ensure that all the inserts will line up with the holes,
especially if you need to drill extra holes in the plate like in this instance.
Scott