Table Calc Totals in LookML

Trying to get an aggregation at specific level - I know this can be done in a visualization easily using table calcs but I am trying to create a parameter that references a % of total. I am assuming it should be somewhat similar to how table calcs are done within the viz (meaure/total.measure) but don’t know how to get the total.measure in lookml. 

a 5
b 10
c 5


need a calc in lookml that does 



b 20
c 20


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