As someone with no voice training, I've been wrestling with this for some time. That is, singing (or buzzing) let's say a concert C out of the blue with no reference. I stumbled upon something last night that seems promising called the Hey Jude method. All of us are familiar with the opening notes sung by Paul McCartney "Hey Jude" which are C and A. I'll be darned that even without much practice I've been able to make this work somewhat consistently. With daily practice, maybe could get close to 100%. Having these notes as a reference may help landing consistently on the correct partial when starting a piece let's say with no reference note. This is more an issue on the Tuba for me than the Euphonium. Just throwing this out there for those encountering a similar situation as a possible tool. Here's to a nice weekend for all!