The following code solves the problem - The code 'as is' will show the workings.  If you would like to use the code set dtEnd to the desired end date value and set the WorkingDays appropriately then comment out or remove the output from the end and return the final datevar (dt) instead

//Replace the created date with the date you wish to use as the end date

datevar dtEnd := Date(2007,8,14);

numbervar WorkingDays := 52; //this could be taken directly from a report parameter or a db field

//calculate the number of full weeks
numbervar iCompleteWeeks := Truncate(WorkingDays / 5);

//To show working
numbervar iActualWeeks;

numbervar iAdditionalDays := Remainder(WorkingDays,5);

//get the day the end date falls on
numbervar iCurrent := DayOfWeek(dtEnd,crMonday) - 1;

//If there is a remainder and if the remainder is more than the current day then this will also cross over a
//weekend so add another Week
//This is a longer winded version to show the working could just alter iCompleteWeeks directly
if (iAdditionalDays > 0 and iAdditionalDays > iCurrent) then
    iActualWeeks := iCompleteWeeks + 1
else
    iActualWeeks := iCompleteWeeks;

//for every full week we add 2 days
numbervar iResult := WorkingDays + (iActualWeeks * 2);

//dt will contain the correct date.  This variable should be returned in the real code rather than the workings below.
DateVar dt := Date(DateAdd("d", -iResult,dtEnd));

//Return the following in the real code
//dt

//To see the workings leave this code active and ensure that the formula cell is able to grow.
"Working Days: " + totext(WorkingDays,0) + chr(13) +
"# of Weekends: " + totext(iCompleteWeeks,0) + chr(13) +
"Additional Days: " + totext(iAdditionalDays,0) + chr(13) +
"Day Of Week (End Date): " + totext(iCurrent,0) + chr(13) +
"# of actual weekends spanned: " + totext(iActualWeeks,0) + chr(13) +
"Total Days To Subtract: " + totext(iResult,0) + chr(13) +
"End Date: " + Totext(dtEnd) + chr(13) +
"Cut Off Date: " + totext(dt);