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It’s a delicate balancing act… it depends on how MUCH you have to overblow. All in the voicing. If you have to overblow a lot just to hit the higher notes, the second octave can be very loud indeed. If it takes too little pressure, the bottom register may be unusably weak. Ideally ( at least to my preference) it should not take very much overblowing to hit the second octave, which will still sound sweet, and not a whole lot louder than the strong, solid low register.
Paul oughta know; he makes some of the best-balanced whistles out there. 
It’s a relative term. The upper octave, as you pointed out, will always be louder, but in some the upper octave is a LOT louder, in some it’s not that much louder – the latter are the balanced ones.
A couple of months ago I saw a local band, and their whistler was playing an Oak. The upper register sounded just as a whistle should sound – but we couldn’t hear the lower register at all. She needed a whistle with better balance between the octaves.
Your comments are very pleasing to me, Paul! I’m waiting with eager anticipation for my whistle!
Robin
me too me too me too !!
not long now I hope 
Paul’s whistles are worth the wait, my friends…worth the wait.
Best.
Byll
ROBIN!!! your getting a Busman!!! God where have I been lately?
OK fine, Im not jealous or anything. Doesnt effect me. No, no sir eey.
Tom
On my water weasel low-G, carefully played, both octaves are in perfect balance using almost the same breath pressure for both. The difference is in the “attack” when the note starts. If I start the note “sharply” it will go to the second octave, if I start it a little softer and “blow up to it”, it will stay low.
The WW seems to “resist” the break in both directions, allowing for an overlap in breath pressure between high and low.
One of the tricks I’ve learned is that to keep everything balanced, I must always be playing very close to the break.
Balance between registers’ volumes is one of the issues of consistency.
Now some whistles are pure up, breathy down or vice versa sweet down, chiffy up. And the character of those too different “flavours” of sound just don’t fit together when playing through from d’ to C# or A.
And, of course, consistency is also an issue of how much one can tolerate the second octave to be out of tune with the first…
Consistency may well be Pandora’s box…
LOL, Tom! If it makes you feel any better, I’m still loving my Clarke original, or rather it is deigning to sing for me. Anyways, I decided a Busman was a better belated birthday present than the car detailing I was promised! After all, which lasts longer and which will follow me through my days? A clean car, never!
Robin
er…if one must be specific,this “balance” is realy an oscillation between both octaves. The voicing sounds a couple of cycles in the lower octave,switches to the upper for a couple cycles and then back to the lower etc. (ad-infinitum) but this switching happens so fast it “seems”, to the ear,to be both octaves at once.
If a whistles labium ramp is very shallow and has a sharp edge, it will be more apt to do this. It is common for Tabor pipers to use this technique.
Good choice Robin. I can`t wait to hear what you think of it. Actually I just recieved my first wooden whistle this weekend. Its a Tony Dixon 3 piece wooden flute in rosewood. I have jumped from the frying pan into the fire so to speak. And I am happy as a clam. And if there is anyone else thinking of making this jump please consider this wounderful instrument. Very reasonable price and not to long of a wait. And they are beautiful.
Tom