Please update the New Mill Tool dialog and online help's reference of Shoulder Length.
Shoulder Length causes confusion because in straight bits there is no shoulder. In this case there should be an option of "No Shoulder" to leave shoulder length the same as body length.
If you renamed Shoulder Length to "Length Tip to Shoulder" it would be a much more accurate description. Currently you disable the OK button to prevent bad data (in case the user measured the start and stop length of the shoulder - which is what the literal definition asks for).
Rather than frustrate users with disabling the OK button simply renaming the field and updating the online help to have an accurate defintion would help.
http://fusion360.autodesk.com/learning/learning.html?guid=GUIDA0197982-0082-45AF-B848-8D62CAF57C49
Current online help states:
Shoulder length:
The tool shoulder length.
this should be changed to:
Length tip to shoulder:
The length from the tip of the mill tool to where the shoulder starts. This is used to ensure your tool has proper clearance. If you are using a straight mill tool with no change in diameter choose "no shoulder" and the body length will be used instead.