Hi, I am trying to make a location centered over a unit, (the location is about the size of a screen ingame), and have 1 overlord at each corner of the location square to indicate the boundary. What would be easiest way of doing this.

1. center the location onto that unit
2. create the overlords with mobile grids

If he's asking how to center the location, he probably doesn't know how to use mobile grids. Unfortunately, neither do I. Someone should probably explain that to him. ;o

The old wiki does a pretty good job: http://www.staredit.net/?p=oldwiki&s=29

Daw Roy beat me to it.

Even though mobile grids are the best method of doing what you wish, they aren't terribly practical. Any other air units could mess up the grid.

Using mobile grids to create 4 units at the corners of a screen-sized location is also going to be quite arduous and as Sac says prone to error if there are any flying units.

This might be a good application for a static grid.

Let me outline how it works:
Think of a x-y-coordinate system you know from math. You represent the the axises with burrowed P11 units along the map border (only top OR bottom and left OR right). Each unit represents one grid unit: For x=1 the leftmost burrowed unit will be given to P10, for x=2 the x=1 unit is given back to P11 and the unit right of it is given to P10 and so forth.
Using a 1x256 (or whatever the size of your map is) location which acts as your x-coordinate cursor centered on the unit you want have overlords around. Since locations can't go outside the map it will always reach all the way down to your x-axis and you can give the burrowed unit to P10. From there you go 10 steps left (screen is 20x12 afaik) and you have the x-coordinate of your first 2 overlords.
Then you do the same with a 256x1 location to determine the y-coordinate.
Where the locations intersect you create the overlords. This, however, isn't an easy task and I can't think of a way to make it happen instantly.
You gotta shoot interceptors (fastest air unit) along 1 1x256 location and create the overlord where it hits the other location. Improve this by shooting 2 interceptors from 2 different coordinates.

If you think this is practical for your map we can discuss this method further.

I've definitely toyed with the above method before, and even designed a map for it. I thought about telling him, but I figured I should document it far better first (which I will do in the next week sometime).

There is a way of doing what you want without screwing up any air units and finding the needed location, but I'm working on the details of it.