I think the pressure built up in the tank will be the same no matter how large or small the office is. 

The fuel leaves the tank at a very small flow rate: something like 5 ounces in 6 minutes.
The crankcase or muffler tap replaces the departing fuel with air or exhaust at a much higher rate.  So the pressure built up in the tank is the same in any case.

If you had a big V8 or a jet engine the fuel leaving the tank might need a substantial air vent to replace its volume, but not so with a mere stunt engine.

There are two muffler pressure taps currently available. One is made by Fox. It has a very small hole and is screwed into a 4/40 tapped hole. The more commonly used one is made by Dubro. It has a larger hole and 6/32 threads. I have used both successfully without any additional restriction. If you wish to try some restriction cut off a short piece of 1/8th brass tubing. Block it up with solder or JB weld then drill it to the size of hole you prefer, usually 1/16" or smaller. Insert this restrictor into the vent tubing, go fly, see if it makes a difference in the engine run.

Here's what is going on, during the main part of the flight the setting is solid uniflow style no problem at all. Once the uniflow vent uncovers for the last 4 - 5 laps the engine goes very lean. Now I expect the engine to lean near the end but not go very lean. I suspect that the combination of the large pressure tap hole and the choke down Fox 35 venture with the ST needle valve assembly cause strong suction from the engine and a little higher pressure in the tank results in a needle setting that is only two turns out. Once the U vent is uncovered the pressure pulse from the muffler can go back up the muffler and reduces the pressure in the tank. That plus the now low fuel push from the circular head result in the very lean run.

To me it seems that restricting the hole size in the muffler tap to just give the pressure to match the engine draw. This should reduce the very lean end of tank run to a normal lean at the end by lowering the initial pressure into the tank thus having less of a total pressure change and requiring the needle to be opened more for the beginning of the run.

Best,     DennisT

The main reason for using the muffler pressure with the uniflow tank is to greatly reduce the "up wind" rich "down wind" lean run you get in higher winds with the open uniflow vent. Some have used directional vents or restrictor pieces to do this. The muffler pressure when it works does this very well. I'm hoping to get the right balance of pressure into the tank to get this benefit.

Best,         DennisT

I think one benefit of the small hole is to limit the amount of fuel that can actually drain into the muffler.  Best practice is to clamp or disconnnect the pressure oline during fueling.  While a tiny hole doesn't reduce the flow of hot exhaust gas, it can limit the gravity spillage of fuel.