Fivestar with half stars and correct average

I'm using Fivestar. My client wants to have half stars. I already changed the CSS, so the widget now shows half stars. I am using 10 stars in the background which show up as five stars (with halves). That part works fine.

But: Under the stars the average is shown. Since I'm using 10 stars in the backround, the average shows e.g. 8 - but I need it to show 4. Any ideas how I can achieve this in an easy way? A way that also works with AJAX-updates?

I tried writing a field formatter, but Fivestar is too complex for me to build my own formatter. There must be an easier way. Maybe a form_alter? Maybe something with preprocessing? Any pointers?

EDIT: It seems print drupal_get_form('fivestar_custom_widget', $values, $settings); is the way to go. I found this Snippet: http://icodesnip.com/snippet/php/drupal-fivestar-custom-widget
Trying to make it work for my case.