The Plastic Practice whistle can be easily made into a tuneable whistle with the help of a 3/4" PVC Coupler.
 

There is however, a proper place to cut it for tuning joint and reasons why it is cut there.

1.  For Low-D practice whistle, measure 435 mm from the bottom of the whistle and cut in half.
2.  Install the coupler.
3.  Play the whistle for few minutes of warm up and then check the tuning.  If it plays too flat, cut off a small amount from the mouthpiece half.
4.  Try tuning again and repeat as needed.  The coupler will allow a certain amount of adjustment in the tuning.  If you need more tuning adjustment ability, simply cut the dividing wall inside the coupler out so that the whistle length can be adjusted to a greater amount.

The purpose of cutting it at 435mm for the body section is so you can purchase other tonebodies that fit the whistle.  The Low-D tonebody on Plastic Practice whistle is 435mm long.  Cutting it here will made the mouthpiece compatible with other tonebodies.