All players will play with different breath pressure and as such the pitch of notes can vary to a small degree. No straight bored wind instrument plays perfectly in pitch on every note ( no matter what any adds for more expensive whistles may say) but the human ear is able to compromise.
If you find you are constantly blowing a note too sharp then it is a simple matter of putting a small piece of electrical tape over the top of the hole below the note you are playing. Highland pipers do this all the time! This will enable the note to be closer in tune.
If on the other hand a note is sounding too flat then you can take a small file and very carefully file away the top of the hole below the note you are playing, this will sharpen the note. It is possible if you are careful to actually ’undercut’ the hole so on the surface it does not look or feel different. If you go too far then use tape as above.
If with your style of playing the bottom note is playing sharp then try putting a small piece of bluetack inside the end of the whistle – this will flatten the bottom note. If it is too flat then you can carefully sand or file a small amount off the end.
Moving the mouthpiece in or out only really makes a small difference and as a general rule with my whistles you probably won't need to move it anyway. If you do feel the need to move the mouthpiece then only a small amount even as little will make a quite a difference pitch wise so there is absolutely no need to pull it out more than 10mm max. I If you pull it out too far then the mouthpiece will not be secure and will 'wobble'. If you need this much adjustment then there is something radically wrong with either your playing or whoever you may be playing with tuning of their instrument. My whistles are set at close to A440 as possible. Any slight difference is not really noticeable to the human ear anyway. Relying too much on an electronic tuner is not always a good idea either as a note or interval in one key is not in technical terms the same as another. In other words an A in the D scale is not technically the same A in the G scale. It all gets a bit complicated but it is based around the idea of tempered or non tempered tuning. If you tune a piano with an electronic tuner then it would end up sounding horribly out of tune.
Contrary to what many people say about a particular whistle being in tune throughout it's entire range it is my belief that with any tube with a vibrating column of air there is no way this can be so. One can get close but it is always a compromise. It quite honestly is the simplest of instruments an must be judged as such. The same can be said about flutes. Even the silver concert flutes are not perfectly in tune. A player needs to tailor their breath pressure and embouchure to suit.
NB: The mouthpiece moves in the tuning barrel! Don't try and twist the body.
Always make small adjustments, don't be too heavy handed. If you are unsure it is probably best to leave things well alone.
IN CHOOSING WHAT WHISTLE KEY here is some advice (probably more for the novice really) on which one to choose:
Low D will give you the same keys as a small D - D and G with A minor, Eminor being the easiest keys.
Low E will give you E and A with Fsharp minor, Bminor.
Low A will give you A and D with Bminor, Eminor.
Low G will give you G and C with Aminor, Dminor.
Low F will give you F, Bflat with Gminor, Cminor.
If you play a lot of highland bagpipe tunes A would be a good one to try and possibly E.
If you just want to 'doodle' around by yourself and your hands are not quite big enough for a Low D then a Low E or Alto F may be a good choice.
When playing the large whistles it is important that you lay your fingers flat over the holes (see below) and use more of the middle pads of the fingers rather than the tips as you do when playing smaller Ds and Cs etc. If your fingers are long anyway maybe it is not so important.
USING ELECTRONIC TUNERS: In short don't!! Well not in auto mode. To explain is too lengthy for here.They are only accurate if the air temperature is 68F. If yours has a sound option then use this and trust your ear. You can also use a keyboard or tuning forks or against another instrument.
THE PIPER’S GRIP
Modified Piper Grip for the Low Whistle
New players of the low whistle will eventually discover that the typical tin whistle grip - playing with the fingertips - leads to the rapid onset of a physiological phenomenon known as 'Numb Thumbs’ and sore and aching hands.
In order to avoid Numb Thumbs, it is necessary to play the low whistle with a modification of the style used by bagpipe players. This is called "piper's grip" or "piping fingering," or whatever you like to call it.
The figure above demonstrates the regular whistle method of finger placement. This Will Result in Numb Thumbs!
Note how our model's hands look cramped and arachnoid. If he keeps this up he will suffer from aching and tired fingers not to mention in-accurate covering of holes.
Low Whistle / Piper's Grip: The best thing since sliced bread for whistle players
Note that the fingers are gracefully and modestly curved and quite relaxed. I would tend to spread my bottom hand fingers even more so more of the middle pads are used. Just an idea
Keep the fingers relaxed. Do not squeeze the tube between the thumbs and fingers. Using the middle joint of each finger will allow the holes to be covered without exerting great pressure, thus avoiding Numb Thumbs.
Some players use the pad of the middle joint of all three fingers of each hand. Perhaps more common is an approach that uses the middle joint for holes 1, 2, 4, and 5 (covered by the index and middle fingers of both hands), and the pad of the first joint of the ring fingers to cover 3 and 6.
Photos courtesy of Chiff and Fipple