The Heimat tone is played on the mouthpiece only. Not on the horn. Whilst I don't know what my pitch is, as I've never really investigated this approach, I do know that if someone hands me my mouthpiece and says "buzz me a note" it will pretty much always be roughly the same pitch. Much like I have a "default" tempo that I will tend to play exercises at if not presented with any other tempo mark. Get your mouthpiece, buzz a tone that's "in the middle", and make a note of what the pitch is either using a tuner or a piano (better). Repeat the next day, and so on. Eventually you will see a trend.