The flow rate is not an exact science so there is no real need to include the skimmer into the calculation. It also depends on where the skimmer is as to if its output has any effect on the acutal tank's current or not. If the skimmer outputs directly into the tank then yes the skimmer would increase the flow rate but there no real maximum flow rate that you dont want to go over so there isnt much need to try to figure out what the acutal flow rate of the skimmer is. Remember the skimmer pump might be rated at 350GPH but once the air and water gets mixed the flow rate is lible to be lower.
If the skimmer outputs into a sump then its flow would have no effect on the main tank as far as adding additional circulation.
