I’ve made some modifications to TWCalc, Tin Whistle Tonehole Calculator
Version 3.2 for Windows
You can download it here: http://www.tinwhistles.us/twcalc.htm
A link to the Instructions are there as well.
You can also download it from this location: http://bingamon.homeip.net/twcalc.zip
The recent flute-diatonic spreadsheet incorporates a 61% tonehole efficiency instead of the 75% used in the original Benade equations.
The original 75% efficiency causes whistles to have a flat pitch on the lower notes and progressively gets sharper as you go up the octave. The new 61% changes this.
You can alway change this in program setup in case your toneholes edges inside the bore are rounded - which affects the efficiency of the hole.
Also, the block to end of whistle length calculation is more accurate than before. Keep in mind though that any geometry variations around the fipple area can and will affect the length of the instrument - which can be resolved by making the instrument tunable.