You need to keep your screen ratios the same.

1680 / 1050 = 1.6
840 / 512 = 1.64
250 / 150 = 1.6667

Change the 840x512 to 840x525.
Change the 250x150 to 240x150 or 256x160.

See if that helps.

I have my UI scaling in wow enabled and set to 1 on everyone.