In essence, " INPERIOD(Date to test, Time period) ". Note that the Time period is optional. By using the INPERIOD you obtain directly a boolean response (e.g., TRUE or FALSE). When using this, alongside the CURRENTPERIODSTART(), (e.g., INPERIOD(CURRENTPERIODSTART()) ), you would be defining when the 1st week of the period took place.
If you combine this, with a similar formula to the one you provided," IF NOT 'SYS01 Time Settings By Week'.'1st Week of Timescale?' THEN PREVIOUS(Ending Inventory) ELSE 'DAT01 Beginning Inventory'.Beginning Inventory ". It should work, while it would allow you to reduce one of your line items.
Hope it helps, please let us know if that is not the case.
Beginning inventory: IF NOT 'SYS01 Time Settings By Week'.'1st Week of Timescale?' THEN PREVIOUS(Ending Inventory) ELSE 'DAT01 Beginning Inventory'.Beginning Inventory
1st week of Timescale?: INPERIOD(CURRENTPERIODSTART())
Please remember that your line items had a slight different name, such as "First Week of Timescale?". Once you amend this - better to enter the formulas manually rather than copy and pasting- it should work.