I got this far but I'm having a problem with the set field as well. Here is my XPath:
../../../my:task/my:tasks[../my:Copying = string(true())][position() = count(current()/preceding-sibling::PM_Tasks) + 1]/../../../my:task/my:tasks/my:Title
The error I receive is ""preceding-sibling::PM_Tasks" does not point to a valid location path of a field or group."