There really isn't easy solution what you're asking jeredwolffe.
If the skin edits the mainbar's size/position somehow then there's problem with the resolutions. I have tried to cower the most used resolutions in my UI but it's a LOT of work to keep all the files up to date, that's why many of the skinners don't directly do this themselves.
The formulas presented on the skin pages are basically the info how you can center the mainbar. I can't remember mainbar's ID now but it's mentioned in various guides here.
To get a custom mainbar centered you need to know your own resolution and as well the dimensions of that specific skin's mainbar (you can check it from the xml or by a picture browser that supports targa files).
Then you can calculate the X and Y values. Actually you don't have to give a damn about the Y value as the UI doesn't allow overlapping so you can just use some high value in Y coordinate (like Y="2000") to make the mainbar to stay at the bottom.
X = (resolution width / 2) - (mainbar width / 2)
for Example you have 1600x1200 resolution and mainbar is 250x160