Select the notes

A whistle is defined in terms of the notes it produces.  These notes are really just frequencies with names. 
(Note: this is an area with lots of complexities.  What I present below is a simplified  representation that suits most purposes.  TWJCalc can cope with  some of these added complexities.)

Chromatic

A Chromatic is a set of notes from which you can choose.  TWJCalc has a built-in chromatic called 12TETA440.  This is a standard in the western world, and if you select notes from this chromatic your whistle is likely to be in tune with other instruments of the western world.  12TETA440 means that there are twelve tones per octave, equally spaced, with the note 'A' defined to be exactly 440Hz (12 tone equal temperament A440).  If you really want, you can define another chromatic with different freqeuncies or names, or both, annd use that in TWJCalc.

Base Note

This is the note that will sound when you close all the holes in your whistle.  It is sometimes called the 'bell note'.  It is also the note that gives its name to the whistle (usually, musicians dislike predictability).

Scale Pattern

A scale pattern defines the intervals (in semitones) between the successive notes of the scale.  For example a major scale is defined as  2,2,1,2,2,2.  This is the most common scale for a whistle.
TWJCalc has several built in scales.  Mostly these will be recognisable to musicians.  There is one that is special for whistles, and this is called 'Major Plus'.

'Major Plus' Scale Pattern

This scale pattern is designed to allow the addition of an extra closed by the thumb of the top hand.  This extra hole allows for a clear and strong note (C natural on a D whistle).
The pattern differs from the standard format by having an asterix (*) indicating that the thumb-hole is not operated in the normal whistle sequence.

Defining your own Scale Pattern

You can define your own scale patterns for use in TWJCalc.  You can only use these in the stand-alone version of the program.
A scale pattern is stored in a file with the .spn suffix.  The contents are XML.  Here is how the Major plus scale pattern would look in a file:

<TWJCalc>
    <ScalePattern Name="MajorPlus" Pattern="2,2,1,2,2,1*,1" />
</TWJCalc>

Having defined your scale pattern, run TWJCalc and choose file - load scale pattern.  Then use the file selector to choose the file.  If you have been successful the new pattern name will appear in the scale patterns list, and you can select it as usual.