Power Pivot :: Calendar Look Up Table - Custom 24 Hour Start Time

I know how to do this in Excel, but not sure about Power Pivot. Our fire department works on a 24 hour shift that starts at 0700 each morning. I want to look up what shift is working depending on the date and time of dispatch. In Excel, I would do a look up table, but instead of using false() as  the last parameter, I would choose true().

I am not sure if this would be best created as a calculated column in the table that holds the incident date and time values, or have a separate table and create a relationship to use the shift in the rows/columns of my pivot table.