Pivot Table Calculated value based on another item

[lukuser]

Hello I need to provide a '% of' type of calculation but based on another item. I guess it's best explained with an example. Here's an excerpt from my Pivot Table (%Comm column was added to the example):

<code style="margin: 0px; padding: 0px; border: 0px; font-family: Consolas, Menlo, Monaco, 'Lucida Console', 'Liberation Mono', 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', 'Courier New', monospace, sans-serif; white-space: inherit;">Agent        GLAccount                        Comm    %Comm
_General     Accounts receivable - Regular    -$250   
             Commission payable Sundry         $120   
Johnny       Commissions earned                $140   
             Commission expense               -$70    50%
Gilles       Commissions earned                $110   
             Commission expense               -$50    45%
</code>

So the column '%Comm' is what I'd like to calculate. It is ABS(Comm of 'Commission expense' / Comm of 'Commissions Earned') for each agent. So, for example, 50% = ABS(-70/140) I'm guessing I'll need to create a Calculated Field '%Comm', but I'm not sure how to define a formula for it.