The L-shaped probe grew legs, so this was improvised with a straight metal tube. There seems to be a persistent notion that angling the probe toward the appliance (into the exhaust stream) is the correct technique, but wouldn't this result in interference from ram air pressure of the rising flue gases?
When testing draft, using the straight probe will be a close approximation of static pressure because the velocity is so low. You should have the end of the probe close to the center of the vent and at a 90 degree angle to the pipe.
Forget the draft measurement issue,,, just look at that incorrectly applied pipe wrap next to the flue.. Surprised it hasn't melted yet. Anyway,,, many who follow the 1200 standard simply test draft using a smoke pen or some visible indication (yes a mirror does work) of proper draft. The old BPI rule of outside temp and draft pressure failure made no sense at all when we could visibly see it drafting under worst case conditions..
I agree that checking for combustion spillage is more important that checking for draft. It is possible to have high draft pressures and spillage at the same time. For example, if a 75,000 BTU water heater is vented with a 3" pipe, you will have high draft pressure and spillage at the same time.