John, there are several gauges on the dwr site that get you the info you are looking for.
Big T at the canyon mouth will get you close to how much is in the narrows after everything else is pulled out.
There is a gauge for the dille diversion as well
There is also a guage for the north fork big T at drake. Main inputs to the big T are the dam output of big T below lake estes and the north fork at drake. Outtakes are dille diversion, and a diversion at the idlywylde dam that I don't know how much is taken out through.
Big T at 570 is a great juicy flow. Its pushy and has a surprising amount of speed the first time you get on it. Yes its got an extra stomp from the moderate 300-400 levels that many see. We got on it at 1000+ cfs this year and it was classic high water non stop freight train. Love it! 3 cheers for big T maintenance.