I totally understand about the custom scale and how the numbers aren't recognized as what they look like to me. It was definitely a mistake for me to use one. I just didn't know beforehand that it was going to mess up the gradebook. It is really inconvenient not to have the half points available. I am grading essays now and I need the half points and that means I have to jump around, not just grade it all in one place. (I do know the different ways to put in comments but I don't like doing it the quick way because I can't see what I'm doing.) But I think this issue is resolved later so I'll just have to put up with it until the school upgrades to Moodle 2.
Ken, I only sort of get what you are saying. I wasn't averaging or dividing anything, only adding them, and I allowed two decimal places at first (changed to one recently for another reason). Now perhaps Moodle divides or averages in the background where I can't see it -- or maybe I am just not getting why 4 is the same as 4.5 etc.
You can be sure that I will never ever use a custom scale again!