Mizrael,

you're right - guessing the right values can be tedious. It's quicker to calculate them.

Look at width and height of your BG image. Divide the larger value by the smaller value. If your image is landscape format, apply the result to U_Scale. If your image is portrait format, apply the result to V_Scale.

Now let's do the offset: take your result from above and subtract 1. When you're done, divide the result by 2. Give it a negative sign, this is our offset. Again, if you're using a landscape aspect ratio, apply to U_Offset. For portrait, apply to V_Offset.

Here's an example for one of my digital camera shots:
Width 1600, height 1200. Landscape, so U_Scale = 1600/1200 = 1.333
U_Offset = -(1.333 -1)/2 = -0.166