Formula help - calculating duration between dates in separate rows of data

This is not an uncommon problem.     You can use a Pipeline to capture the Date Created of the new record and write it to the prior record.   

In your example when RID 2 is created,  the Pipeline would search for RID 1 and then write the value of Step Date from 2 to Step End Date in 1.

Now you just write a Formula Duration field in each record to calculate the difference between End and Start.