=IF(E:E = "Story total hours","",
        IF(F:F <= 'Work Breakdown'!D50, 'Work Breakdown'!C50,
        IF(F:F <= 'Work Breakdown'!D49, 'Work Breakdown'!C49,
        ...

=IF(E:E = "Story total hours","",
        IF(F:F <= 'Work Breakdown'!D51, 'Work Breakdown'!C51, // Incremented C50 --> C51.
        IF(F:F <= 'Work Breakdown'!D50, 'Work Breakdown'!C50, // Same