In my understanding, you want some initial numbers in the very first period of your timescale.
For that, I would use a separate module with no time scale but the rest dimensions are the same. Then edit your formula to have an IF THEN function where you would refer to this new module if a period is the first one.
The formula should be like this:
IF START() = <a new line item with initial period> THEN <new module> ELSE PREVIOUS(Ending RTR Balance)