As I'm sure you've seen, the "on topic" page states that it is on topic to:
ask a question about ... practice or performance technique.
The same page states that it's off topic to ask about:
recommendations of pieces to listen to, study, or play.
At first glance, it sounds like your question straddles the line between these two, but after some reflection, I think your question is on topic, because I think your question is more along the lines of "how do I improve my tuning" than "which exercise is best?" The first question ("How do I improve my tuning") is about playing in tune, which is a practice or performance technique. Additionally, this seems like the sort of question that would solicit high quality answers from experts who have a lot of experience working on their tuning. And expert participation is what the site wants. So perhaps you could phrase your question something like this: "I play french horn in a concert band and occasionally am not as in tune as I would like to be. How can I improve my tuning? Are there exercises that help, and if so, what are some examples of these? Are there specific things to look for in my breathing, etc. which cause out-of-tune playing?" Maybe this is too many questions for one post, but based on other stackexchange sites, I think it would be appropriate.
EDIT: here's a music.se question that reminds me a lot of your proposed question. This existing question hasn't gotten any flack.