Hi Diane,
It is my experience as a Moodle Support person that the scale has 3 stops: Not Satisfactory, Satisfactory and Outstanding.
Outstanding (Real) yields 100% (percentage) and A (letter)
Satisfactory (Real) yields 50% (percentage, halfway there) and F (letter)
Not Satisfactory (Real) yields 0% (percentage) and F (letter).
The middle grade in a 5-stop scale would also yield 50% and/or F.
A suggestion would be to create your own scale with 2-stops. First stop wouls be Not Satisfactory for 0% and F. Second stop would be Satisfactory for 100% and A.
Hope that helps. take care, Tom Schillaci