How do I fetch value from similar record

Hi , Could you please help me to populate the value for Calculated_implentation_date.

Logic as 

if the implementation_date exists,
then implementation_date
else 
for that assumption_key, get the implementation_date for phase 1 and do this formula

AdjustYear([implementation_date],([phase]-1)



So my final answer should have the Calculated_implentation_date as below

10 2 <null> 05-30-2019
10 3 <null> 05-30-2020
10 4 <null> 05-30-2021