To loop back on this one, the following field set up works for predecessors
[estimated start] = work date
[estimated finish] = formula - date (using end date formula builder)
[estimated duration] = numeric
[actual start] = date
[actual end] = date
[actual duration] = formula - duration
[calculated end date] = formula - work date (as below)
If(
not IsNull ([actual finish]), [actual finish],
not IsNull ([estimated finish]), [estimated finish])
NOTE: the formula for calculated end date shows a red line and yellow highlight error at [actual finish] but still it works anyway. Also, if editing this field it will sometimes pop up a syntax error but this isn't consistent and the formula works anyway.
I need to start using this with actual data and see if any other issues pop up.
Thanks to Esther above and Lovell from Quickbase who helped with this.
