Return summary task name for custom field

I'm trying to create a formula for a custom field that enters certain text into the field depending on the name of the summary task that the task is nested in.  For example, if the task is nested in the summary task 'Business Plan', then I'd like the custom field to have a value of BP.  I believe my only question for this is how I return a task's summary task name for my custom field formula.

Thanks.

jjmclell