Pretty good again, but the reason you are having to overexpose all your shots is that the white background will fool the light meter, its looking to get that white exposed so that it isnt burnt out, and as the background could be up to 80% of the photo it will often ignore the mini as it takes the larger area into account. Easiest way to fix this is to use a coloured or 'fade' background.
Also, F16-22 will give a huge depth of field, but it might stop you from blurring the background, if usually find F8 is a wide enough apeture for a normal 28-30mm mini.
Also SaxonAngel's comment about setting the white balance is absoloutely crucial. Your camera's manual will explain how to do this, most can do a custom white balance, for which you will need a 'grey card' which is only a few $s from a photography store, and well worth getting for the results.
Also i would maintain autofocus is best if you can get it to focus on so small a subject, it may not if you mini doesnt fill the frame.