Answer
Rather than hardcoding a static placeholder default value, which skips the real auto-calculation routine entirely and leaves the field literally at that static value, configure the field's Default Value setting to instead CALL the relevant calculation function itself, passing through whatever parameters that function needs, typically referencing other columns/fields already present in the same migration record. This way, the Migration Job actually invokes the same underlying calculation logic the UI would normally trigger, correctly populating the field with a genuinely computed value rather than a static placeholder that has to be manually corrected afterward.