Why would you want to drain the UAT on a test stand?? Once is is full, and all the bubbles are removed, you should leave it full of fuel. Once you see bubbles coming from the main tank, that is the end of your useable fuel, and you should shut down....

A couple of ounces left in the main tank is not bad..it depends how effective your main clunk is in picking up that last bit of fuel off the bottom of the tank.....