cheip wrote:
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!