Hi there,
A friend of mine is trying to make a low C whistle after making
successfully a Low D …
Problem is that he has done Low C whith the same embouchure than the
Low D and also with proportional measurments : interpolation from D
to C … But the sound is not as clear as for the low D … it less
metallic with less backpressure… less presence
Question is, what should be taken into account to assure the
transition 
the tube used is aluminium :
for LowC :
ext 28mm
in 25 mm
length approx 650 cm
for LowD :
ext 25
in 22
Length 588 cm
Cheers, 
Nicolas.