In the definition of Letter grades in gradebook, there is a selection box like
Grade: A

• 93% -
  Grade: A-
• 86% -
  Grade: B
  ...

There are two issues with it:

• decimal boundary like 66.67% can not be set
• the boundary value itself automatically leads to the better grade (eg 93% give A) only

The required behaviour is that both of the following use cases are available:
if grade > 93 then "A"; if grade <=93 then "A-"
if grade >= 93 then "A"; if grade < 93 then "A-"

with the value precision up to 5 decimal points as we have everywhere in gradebook.