Hi there, I had some question about making my own pennywhistle, I have actually made a few, but I was always copying hole spacing, and size off of another whistle, here are my questions.
Is there a formula for determining hole spacing?
How do you decide how big to make the holes?
When tuning the whistle, which part of the sound hole is material removed from to get the desired sound?
Any help with these questions would be greatly appreciated. 
There are a few whistle calculators available. Most aim to get the first octave in tune. Hans Bracker has posted one based on Pete Kosel’s Flutomat: https://music.bracker.co/Music/Whistle-Calculator.html, and you can download https://sourceforge.net/projects/twjcalc/ by Phill van Leersum.
If you’re willing to put in some extra work, WIDesigner can help you get all the notes in both octaves in tune. You can download it from https://github.com/edwardkort/WWIDesigner/releases/latest, and read about the process of tuning a whistle at https://github.com/edwardkort/WWIDesigner/wiki/Optimizing-a-Whistle-or-Flute-Design.
Thank you, I will take a look at those.
Here is a little simpler formula:
100% is the length of the tube from the edge where air is being split to make sound.
(Image Source: Mark Shepard’s Simple Flutes book: http://www.markshep.com/flute/books/Simple.html)
A whistle is basically the same as the flute. If I am not mistaken, the “0%” for a whistle should be measured from the wind-blade. But don’t quote me on that. 
I’ve simply done more with making flutes than whistles.
When I make flutes, I use bamboo and have a special tool for making the holes (I could talk more about that if desired). But I basically tune the flute without any finger holes and then start with the lowest hole by making a small hole. Starting with a small hole you can decide how much large the hole should be made in order to be in tune.
Sizes and location of the holes may also change for the sake of ergonomics and sound quality. So it may need to be something you experiment with or read more about.
You might glean some information from here:
http://www.ggwhistles.com/howto/
Thanks everyone. Yes AaronFW I would love to hear any information that you have, I would like to start making and playing sideblown flutes too, and I may have some questions on that later.
I will email you some information that I have.
What materials do you plan on using? Most of my experience is based on using bamboo. I thought it would be worth asking here because if you plan to use a different material, there might be someone else who may have additional advice.
I am mostly using wood, but I do know where some bamboo is, although I don’t know about the quality. I work in the tree trimming and removal business, so I have access to some really beautiful and sometimes rare woods, which is great.
That sounds like a good gig (as it pertains to getting wood that could be used for something else)–way better than harvesting randomly growing bamboo or reeds from people’s yards or train tracks. (Which is largely what I did.)
As it pertains to working with wood, you might also benefit from this topic from the flute forum: https://forums.chiffandfipple.com/t/flute-making-apprentices/97804/1 . The topic was originally about flute-making apprentices but then became about reamers that are used to make Irish flutes have a conical bore.
One thing that I mentioned in that post, that also seems worth mentioning now, is that it is valuable to buy a whistle or flute that is of good quality and inspect it to try to understand what makes it good quality.