The answer it is in the code:

if (score > -0.05) return 'S'

if (score >= -0.15) return 'A+'

if (score >= -0.5) return 'A'

if (score >= -1) return 'A-'

if (score >= -1.5) return 'B+'

if (score >= -2.5) return 'B'

if (score >= -4) return 'B-'

if (score >= -5.5) return 'C+'

if (score >= -7) return 'C'

if (score >= -8.5) return 'C-'

if (score >= -10) return 'D'

else return 'F'

• The score is the mean squared error for yes and no bets times -100.
  The score is a negative number, and it's designed in such a way that the higher the score (closer to zero), the better.

• It only includes yes/no bets

• If the green triangles is above the line you get zero score added, if it is below the line you get a negative score proportional to how far it is from the line. Analogous for the red triangles. These scores are averaged if I understand the code correctly.

to answer your second question: https://manifold.markets/Spindle/calibration

I feel like I've seen an F before too

Spindle’s calibration

Personal calibration results