50000 isn't the maximum range of the stick(The internal value is a 16bit integer, which tops out at 65535), so symmetrical values would be 15535 and 20535.
As a side note, while the internal value is 16bit, most controllers aren't anywhere near that precise; The XBox 360 controller only provides 8bit resolution(only 255 different values) IIRC.
This means that the value will jump significantly, as those 255 values are "mapped" onto a range from 0 to 65535.
I do believe the Z axis would be the triggers on an XBox-style controller, though you can verify this by opening "Devices and Printers", right-clicking your controller, selecting "Game Controller Settings" from the context menu, and double-clicking your controller in the list(These steps work in Windows 7, but may be slightly different in other versions of Windows).
Now you can move the different analog controls and see which axes they correspond to.
If you want to see a live readout of an axis, you can have VoiceAttack run a loop that outputs one or more joystick token values to the log:
Start Loop While : [{EXP:1=1}] Equals '1'
Write '[Blue] X:{STATE_JOYSTICK1X} Y:X:{STATE_JOYSTICK1Y}' to log
Pause 0,05 seconds
End Loop
I added a pause because while VoiceAttack can process the values quite fast, the log cannot be updated nearly as quickly, so running it unrestrained can result in VoiceAttack becoming less/unresponsive.