Status and matching state : "cancelled" or "not applicable"

[Chloé Oternaud]

Hello,
I am wondering how I can improve the use of the % done on tasks for our team :
Some of the tasks are not billable, some are cancelled, and/or we don't want them to be taken into account when looking at the charts % done

So my idea was to have a "N/A" status, that I would like to map to a NEW state in Ftrack . 
Curent Ftrack states are : Not Started, In Progress, Done, and Blocked. (All enter in the computation of the % done.)
I'd like to have a "Not Applicable" or "Ignore" state. This state would be ignored in the computation.

Alternatively, I'm open to any suggestion / workaround
Thanks