I’m a bit confused trying out TWCALC. Guido’s guide shows a D whistle with a 264 mm length, window to end. That’s the length that I successfully used following his instructions. TWCALC shows a D Whistle to have a cork to end length of 287.13mm. If the window is 4 mm tall, the TWCALC window to end length is 20 mm longer than Guido’s! Am I doing something wrong or not understanding something? Thanks.
The dimensions in Guido’s guide say they are guidelines… I made a C last night and ended up with a window-to-bottom distance of 284 mm (his guidelines say to use 300mm). That is over 1/2" difference… He also says that tube thickness plays a big part - that is what my “problem” was as my tube is thicker (and my inner diameter is wider) than the one in his instructions.
Hey, it’s just PVC pipe and a few minutes of your time - have fun and don’t worry about getting your first one perfect!
Oh yeah, I use a tuner to check the pitch…
Good luck and have fun,
Pat
The tubing ID is a huge factor. The tubing thickness will matter as well. And the windway and window dimentions affect it too, as does air temp. In a test I did when determining my tuning slide length, I found that the difference in length pf my Every hight D between 80F and 50F was more than 3 mm. And, when you narrow the windway by operating the tone ring, there is a change of up to 4 mm.
The point being, there is no “right” answer for you until you make one, or two or three and settle on a desigh, you won’t know how your decisions affect the length required.
Some science yes, but some art as well. Of course it is possible to calculate all these things, but the complexity of measuring it all makes it simpler to just have a go. Call it Newton’s method of whistle design if you like.
Or, like most physics classes, simple in theory - more complicated in practice. 
Plunk111 is right on the doubloons, as is Carey (of course). Guido’s plans will work quite well given the same tubing is used and all the measurements are adhered to. And I should add that Guido makes darned good whistles too.
TWCalc is a good tool also. The calculations have a few variables built into them. If your construction matches the variables, everything works out. One measure of advice I can offer in using it is to do your calculations, fashion the head and then cut the tube long. Next you shorten the tube in increments until you sneek up on the fundamental note. Once you have that, drill the other holes using the measurements from the bottom of the tube. You’ll get pretty useful results that way. You’ll still find yourself fiddling with the results unitil your happy and , as Carey noted, you settle on a design.
It’s all a lot of fun in the end.
Feadoggie
Amen to that Feadoggie! I use TWCalc when I find a strange bit of pipe on my workbench. It gets one pretty close. All hail TWCalc! (If you use it, consider paying Daniel for it. )
To save some time and effort, once you have the first go, record which holes have to move which way, and tape over all your holes and drill new ones on another side of the tube. In fact, planning to need two or three iterations, make your first set of holes on the back, and toot upside down. (I mean hold the whistle upside down 
)
The taped holes will add some volume (the physical kind) to the bore and might have some perturbation effects, so don’t worry about tone now. Just get the holes in the right place. The next one without all the swiss cheese will be closer to what you want, but you may or may not need another tube to get it right.
Certainly, and better still, become a customer. And if you are a DIY whistle maker, buy his whistle builder’s anthology. That’s got useful hints throughout.
I make whistles from high E down to low C and while TWCalc is not the only tool I use to design them, it is one of the most helpful. Daniel is to be commended.
Feadoggie
I should be able to make a few tenor whistles next week. I’ll be using 20mm pipe (for soprano whistles I use 16mm pipe), and I expect to get new measurements for whistles in the keys of Bb and A. I’ll publish new diagrams as soon as possible.