When I’m working out the tuning of a whistle, I check the intonation of each pitch, making adjustments to the tonehole configuration until I’m satisfied that I have it as close as possible, with a smooth continuity of the amount of breath needed to blow each successive note on pitch.
Then, I watch the tuner while I play the whistle normally. I may see that one or more of the pitches is different from what I found when I was checking them one by one, which would be due to differences in breath pressure between playing one note at a time and playing many notes quickly in a tune. I then make further adjustments until the whistle plays on pitch during normal playing, rather than the artificial test of checking pitches one at a time.
When I do this, I also make decisions about how much breath sounds the best for each part of the whistle’s range. I may choose to have it play on pitch with a little more or less breath, if that is what best accesses the “sweet spot” of that particular range of notes.
Differences in breath pressure can change the pitch of a given whistle or note significantly, so there will be individual variations for different players.
Awhile back, I had two accomplished players check the intonation of two whistles with identical tonebodies, using software that mapped the pitches statistically during normal playing.
I observed that the two players had somewhat different intonation configurations, such that you might look at one or the other and think, “These notes here are a bit out of tune,” but looking at all the data, the conclusion was that the tonebodies were as in tune as they could be, and from there it would be up to each player to work out how to approach the way the whistle responds to being played. You could see that one player tended to blow a bit more strongly through a certain part of the range, and this was pushing those pitches a little sharper than the other player.
Playing a whistle is like singing in the sense that you have control of the pitch and can play/sing what sounds right to your ear, assuming the whistle is set up properly. This is so automatic, most people don’t even realize they’re doing it.
Best wishes,
Jerry