It really depends on what aggregration strategy you want to use. From our experiences only sum of grades works with bonus points, and it does work well. If you want a percentage for the category you can edit the category item to make it show out of 100 points (100 as maximum) and moodle will show this correctly. The only thing you need to do is create your 'bonus' item as out of the amount of points required to make up the extra 0.6%.
Extra credits do NOT work with percentage based aggregations at this point in time as far as I have been able to tell.
hope this helps. It is not a solution... I think there are LOT of people that would like extra credits to work 'out of the box' with percentage based aggregations.
Moodle 1.9 bonus points
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