The quick answer to the "average" best place for the cork on the Graz original Hotteterre flute is 22mm from the centre of the mouth hole to the cork. However this can vary with each player.

You probably know this: The cork is movable as its position partly dictates the trueness of the octaves, when overblowing to the next register, and as a general rule, pushing the cork further away from the mouth hole tends to narrow the octaves, flattening the high register, and pushing it nearer sharpens the high register by widening the octaves.  The player will want to adjust the cork to play true octaves, and your embouchure is an important part of where this setting is. The placement of the cork in any flute depends upon the whole playing instrument, that is, you are a part of the complete instrument, and the flute is another part. Your individual embouchure and air jet are individually unique, and a cork position that works for one player may not necessarily work for another player on the same flute. So that is why corks are adjustable, either by being pushing them with a stick, as is the case in the Graz Hotteterre, or by the later invention of the screw cap.  It would surprise you how many experienced players never touch the cork, and put up with a poor setting for years, only to discover that a small change improves their playing in seconds.  Of course, it helps to have a good flute, as some flutes will never allow good octaves, so bore shape is super important. Remember also that there is no such thing as a perfect baroque flute, only better or worse. I often quote Philippe Alain Dupré, holding up an excellent flute of his own making in a master class and saying, "This flute is not in tune! Your job is to play it in tune." A good flute will allow you this flexibility, and a poor one will resist all attempts. So you have to set the cork to get as good octaves as the flute is capable of, and then use your playing skills to pull everything into line.

With all of that said, turning back to the Graz Hotteterre original, the top tenon of the middle joint has been squeezed in, due to the thread wrappings over the centuries, and this squeeze is near the nodal point for the A octaves, so these are very narrow, and out of tune. I am going to presume that your "copy" has corrected this 'ravage of time'. The Graz, slavishly copied, would have many problems, yet it is still an important flute to be studied. I would put the playing pitch of the original, for my lip, at about A398.

best wishes!

Rod