Accuracy when averaging text based grades

I want to average text based grades that range from E- through to A+... There are 15 different marks in this range. I have tried changing the percentages and it has helped a bit but I'm still getting some inaccuracies... For example... A student who has B-, B-, B+ is being averaged out as a B-... Etc.. Most of the class is accurate but I have 4 that just aren't quite right... Always a bit lower than it should be. Any suggestions?

It depends on the distance between marks, you will find a 'Round up value' parameter in the text based grade configuration. Try with a small value first 1 or 2

Thank you. That worked