return the scheduled start date from one task to a custom field for all tasks

Hi all

I am trying populate an entire custom task date field with the scheduled start date of one of the tasks. I thought the easiest way would be to:

1) identify the date I need with a custom task flag field set to 'yes'

2) create a formula to look for the 'yes' flag in that field and populate the custom task date field with it.

This worked only for the task where the flag was set to yes (task #36) and the rest of the 49 tasks returned an error message. I'm unsure of how to set this up so that the date with the 'yes' flag populates in this column on all 50 tasks.

This would also need to scale for situations where there were more or less than 50 tasks.

Thanks in advance...

Using MSP 2010 Pro