When you set the barrel valve leakdown, you're setting the size of the "leak" in the barrel valve.
The nozzles have no play in that whatsoever...you're not measuring the flow through the nozzles...JUST the spool in the valve.
In fact, if the nozzles are too small and create too much restriction (on a gas setup), sometimes it's necessary to remove a nozzle line to remove them from the situation.
So, you could adjust the leakdown just fine off the car, disconnected from the fuel system completely.
Spud