Our dear friend Tara had sent me a letter asking about some didge crafting specifics and after I wrote a long and wordy response my computer failed to send it properly and nothing I could do would bring it back up so that I could just send it again, but I could still see it, so I transcribed it by hand and decided I may as well share this info with everybody. The main portion of her questioning was about lengths and interior dimensions as far as tuning and playing characteristics. So here goes....
As far as lengths and diameters, I'm sure that the sticks you have already can give you a bit of a tutorial on what already seems to work. Basicly, air flow is very similar to water flow so in your mind if it makes it easier to conceive of how water would behave inside a boreshape then that gives you a conceptualization to work with. Pitch is basicly a result of speed. In a top end when the input (volume or amount) is the same, air or water will have to move away to make room for the continued input so in a tight top end the flow rate will be faster resulting in a higher pitch AND higher backpressure and in a more open top end the flow rate will be slower resulting in a lower pitch AND lower backpressure. If the bore is conical and opens up too quickly causing a very short high pressure zone near the mouthpiece followed by a low pressure zone where it opens up the effect will be similar to if the didge length is cut near the beginning point of the low pressure zone, so effectivly the key will be determined by the length of the higher pressure zones. Opening up with a conical bore causes incoming air at the bell which gives a bouncy responsive type of playing action. If the bore remains cylindrical without coning out the effect of the backpressure will be more a result of the air inside the column getting in it's own way thus causing a slower flow (lower pitch) and slower response which is in direct contrast to the quick bouncy response of a conical didge. This cylindrical aircolumn causing it's own flow resistance is another way of adding backpressure yet as I described, it's a different type of backpressure in terms of the response and playing characteristics. A bell is basicly a no pressure zone and can be completely cut off without affecting the key but it would affect the tonal qualities as the bell section acts as a natural amplifier which adds bass tones and volume. A bell is a double edged sword as even though it adds to the power of a didge it also muddles the clarity and precision of the upper asnd mid-range responses. For vocals a more open top end projects with more clarity because the movement inside the bore is not as turbulent, though on an open topend you need to keep a longer section of cylindrical bore so that the backpressure remains high enough for ease of play. A tight topend on a conical bore is more suited to a quick percussive style yet the high backpressure also allows the player to nurse it slowly for airplane ( or meditative, or whitefella) style making it easy for a lazy player to do basicly anything they want with it. This is a style I create most often = awesome stick for beginner through intermediate. My opinion is that your abilities (Tara) and your diaphram strength would allow you to push on a didge with lower backpressure than others would find comfortable so if you're crafting for your own self there's no need to go too tight on the top end unless you're trying to get a first trumpet that comes in at less than an octave above the fundamental. You'll notice that didges with a cylindrical bores havea first trumpet that comes in at way more than an octave jump and conical bores will have trumpets that come in closer to one octave above the fundamental. Simply knowing that bit of info there can help you to craft sticks in which not only your fundamental is intentionally tuned but also your trumpets can be intentionally tuned. Like I mentioned about the pressure zones causing the pitch, you'll get the biggest bang for the buck in the tuning processes at the mouthpiece end of the stick by either cutting shorter (raises the pitch,and generally leaves the trumpet about the same depending on what bore characteristics you're getting closer to by this shortening) , hollowing out (lowers pitch and pushes trumpet further from fundamental), adding length(lowers pitch and can draw the trumpet closer, though generally leaves the distance between fundamental and trumpet about the same), or shrinking the interior diameter (raises pitch and brings the trumpet closer). This is all a very basic description. Your mileage may vary.