Measuring across corners or parameters, then equation using results

I would like to calculate one value using measured values from several runs of the same circuit using different parameters.

The calculation is an arithmetic equation, not just an average, max, min, stddev of the measured values.

For example, result 1 - result 2 - result3

I have tried this by making each run its own corner, but I don't see how to write expressions to grab the 3 values, and then an expression to do the math.

I have tried this by making each run part of a Global Variables sweep, but I must use Grouped Parametric Sets, and again, I don't see how to write expressions to grab the 3 values, and then an expression to do the math.

It seems like I need the following:

1. A version of the 'value' function that has a way to specify either a Corner or choose amongst the combos in a Grouped Parametric Set.

2. A 'post all sims'  calculation using other measurements ... I think this can be done using an EvalType other than 'point'.

Is there a best way to do this, perhaps I am missing something easy?

(tool version: IC6.1.8-64b.500.7)

Thanks.