Welcome to the Forum @Darren_Webber and thank you for reaching out!
As it stands, there isn’t a built feature to achieve this in Asana. There is an existing thread in the #productfeedback category requesting this functionality: Setting Dependent Task Dates Based x Days From Completed Task. I would encourage you to support this request by adding your vote to it.
With that said, I would recommend you to have a look at the 3rd party integration Flowsana (developed by our Forum Champion @Phil_Seeman) that has this capability.
I hope this helps Darren but please let me know if you have any further question!