Found it

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Basically it's a value between -1.0 and +1.0 which is computed thus:

e = (k - d) / m

Where:

- k is the total kills minus team kills
- d is the total deaths plus suicides
- m is the total fractional minutes played

The grades for this are:

>= +0.65 = A
>= +0.3 = B
>= zero = C
>= -0.2 = D
>= -0.4 = E
>= -0.7 = F
>= -1.0 = X

We might tweak these thresholds in future but that's what they are for now. Mathematicians amongst us may notice a degree of error in the formula, in which case the yield is limited to the specified range. Observers may also note that the more you play, the higher chance of degradation against your grade.

It is a new way of representing an overview of skill for profiles so constructive comments welcome.
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I'll link you to the post if you wish, just ask.