Try these five formulas:
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[Due Date 1] formula:
var Number NumDays = ToDays([End Date] - [Start Date]);
var Number IntDays = Int($NumDays / 5);
var Number RemDays = Rem($NumDays, 5);
[Start Date] + Days(If($RemDays = 0, 1 * $IntDays, 1 * ($IntDays + 1)))
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[Due Date 2] formula:
var Number NumDays = ToDays([End Date] - [Start Date]);
var Number IntDays = Int($NumDays / 5);
var Number RemDays = Rem($NumDays, 5);
[Start Date] + Days(If($RemDays = 0, 2 * $IntDays, 2 * ($IntDays + 1)))
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[Due Date 3] formula:
var Number NumDays = ToDays([End Date] - [Start Date]);
var Number IntDays = Int($NumDays / 5);
var Number RemDays = Rem($NumDays, 5);
[Start Date] + Days(If($RemDays = 0, 3 * $IntDays, 3 * ($IntDays + 1)))
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[Due Date 4] formula:
var Number NumDays = ToDays([End Date] - [Start Date]);
var Number IntDays = Int($NumDays / 5);
var Number RemDays = Rem($NumDays, 5);
[Start Date] + Days(If($RemDays = 0, 4 * $IntDays, 4 * ($IntDays + 1)))
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[Due Date 5] formula:
[End Date]