Ben, I usually get it close within octaves and call it a day. I have used several apps and they usually work OK. I do have a small chromatic tuner that works better than the apps as far as consistency but like I said most apps work fine. One thing that I have found is that if you just blow into the melodica you will always blow different, What I do is fill my lungs with air and release it as even as possible. Try this and then just blow and see if that makes a difference for you, also don’t forget to ping the reed after scraping. One more thing, make sure that there are no other sounds while you are tuning, a fan, air conditioning sound, background radio/TV noise, kids. even faint sounds can effect the tuner.
hope this dose not confuse you any more 🙂