fast formula to get assignment start date when action code gets changed to promotion

Summary:

I have written a formula to extract assignment start date when action code = promotion, but it is getting into loop Iterations. any idea how to correct the code?

Content (required):

DEFAULT FOR HIRE_DATE IS '1951/01/01' (Date)

DEFAULT FOR LATEST_START_DATE IS '1951/01/01' (Date)

DEFAULT_DATA_VALUE for PER_HIST_ASG_ACTION_CODE is 'NULL' 

default for PER_ASG_PERSON_ID is 0

DEFAULT for PER_ASG_REL_ORIGINAL_DATE_OF_HIRE is '4712/12/31' (date)

DEFAULT FOR PER_ASG_REL_DATE_START IS '4712/12/31 00:00:00'(date)

DEFAULT FOR PER_ASG_EFFECTIVE_START_DATE IS '1951/01/01' (Date)