Redmine

I have 2 custom fields that need to be dependant on other fields.
First is the due date. The due date is based on the category and adds x number off days to start date.
Second would be a custome field that needs the project + custom field(device) + a # this(prefer the number that comes after the tracker). This field must be unique.

Any help on this is greatly appreciated. I am new to Ruby, Ruby on Rails and Redmine.