DeltaDown = Ldown - Lidle, DeltaUp = Lup - Lidle, DeltaMixed = Lmixed - Lidle.
Final grade by MaxDelta = max(DeltaDown, DeltaUp, DeltaMixed).
Scale: A+ <= 5 ms, A <= 30 ms, B <= 60 ms, C <= 200 ms, D <= 400 ms, F > 400 ms.