surface integration (doing calculations with derived values)

Hello,

I have a simulation with several subdomains.
In the postprocessing, I'd like to relate two of those subdomains, i.e. I want to compare the power flow of say domain 1 to the power flow of domain 2.
The power flow values for the respective domains are easily obtained by chosing derived values - surface integration.
But how to do some calculations with these values without resorting to a pocket calculator?
Can I tell Comsol to calculate the ratio (quotient) of these two values?

Thanks,
/H.