Hi all,
I have a keyless flute without the screw cork adjuster in the head. With my modern-concert flute I am used to check the position of the cork in the head and eventually regulate it.
How to check if my cork is in the right position? If not, how to regulate it without the screw?
Thank you! 
https://forums.chiffandfipple.com/t/question-for-seery-owners/56395/4
While the headjoint of a Boehm flute is conical/parabolic (& cylindrical body), an Irish flute is the reverse: cylindrical head/conical body. You can move the cork using a dowel sized to fit into the headjoint and push the cork from either end.
There are more experienced heads on this forum, but just to keep the ball rolling: Iāve been told the place to start is the diameter of the inside of the headjoint should equal the distance from the center of the embouchure hole to the cork. (I believe it was Martin Doyle who said that on a video on his website, but my memory may be foggy.) After that placement you may need to adjust minutely to your playing style and the particular flute you are playing. This is waaayyyy easier with a threaded cork. But if you donāt have one get a dowel stick that fits inside the headjoint without much play. If your stick it too small you may just end up denting your cork. Gently push the cork to adjust. This will likely drive you a bit crazy of you have a lot of back and forthing to do. I have used a dowel from a hardware store I was too lazy to cut down, so I would place the end of the dowel on the floor and push the headjoint I had firmly grasped in my hands. Most people would likely cut the dowel down and use a sturdy table as a surface to push against. Work with an electronic tuner. I found cork placement did affect the pitch of the octaves as well as the fluteās ability to play the scale in tune with itself. One of my flutes loves 21 another 19. If you are switching from Boehm to wooden flutes it pays to keep in mind that even the best trad flute out there will need a little more work out of the player to keep notes in tune. It is one of the reasons they fell out of fashion with orchestras back in the day. I had one antique flute that would not budge no matter what I tried. I took it to a band instrument repair person who whacked it out with more force that I had confidence to use myself. LOL
Ok thank, now I know hot to measure the stating distance!!! 
Ideally, you want the first and second octave to be in tune with each other, I check the A in both octaves. If it is a Rudall design, it will be likely that it will play best around 21mm like Kevin said. Pratten flutes will not require it to be pushed out that far.
Remember the farther the distance the more the second octave will flatten, this is usually the case that the second octave will be sharper.
I would not set it father away then 21mm as it will sacrifice the tone.
Good luck!
My āoriginalā problem is that the first octave C# is a lot flat, also on my ābackupā flute is a little bit flat, but not so much. I will cehck the octaves, but the yare not my principal problem, the C# is the problem.