I drilled the hole, used a piece of copper tubing, soldered the supplied fitting to the end of the tube to adapt it to the Manometer's hose.
I think the fitting would work fine if threaded into the flue pipe, but may get clogged with fly ash over time. It wouldn't be difficult to clean it out, just poke a wire through the fitting.
I remove the copper tubing ocassionally and tap it to knock off the fly ash, and reinstall it.
Welcome to the forum, Hope this helps with your question.