Column Status Formula

Hi Everyone, 

I have a status column with the following options: Not Started, In-progress, Complete, N/A.

I've created a formula if the parent row is set at 'Not Started' then the sub-task would automatically change to 'Not Started'.  Same with Complete and N/A.

In-progress needs to work a little differently. The parent row may be in-progress where a sub-task may be complete or N/A. 

When In-progress is selected it changes all the sub-tasks to blank. Is there a formula that would allow the sub-tasks to remain unaffected if the parent row changes to in-progress?

Best,

Ashlie