I contacted Autodesk asking how to change this and here is a excerpt to the response I received.
You are correct that you cannot edit the actual tick mark, you can only select a different type of mark from the available list. If you wish to request an additional tick mark option, please submit your requests using this form:
Interesting.... so I submitted a request and we'll see if the next release we have more options to choose from... I'll keep you posted.