If I have understood you correctly, you can loop the Number of Days
every 5 days (weekends excluded) and add it to the original Due Date
.
And the formula should be:
=([Prepare Date]+[Number of Days])+ROUND([Number of Days]/5,0)*2
Reference: ROUND function.
UPDATE:
Here is the updated formula:
IF(AND(WEEKDAY([Prepare Date]+[Number of Days],2)<6,[Number of Days]<5),[Prepare Date]+[Number of Days],
IF(WEEKDAY([Prepare Date],2)=6,[Prepare Date]+[Number of Days]+ROUNDUP([Number of Days]/5,0)*2-1,
IF(WEEKDAY([Prepare Date],2)=7,[Prepare Date]+[Number of Days]+ROUNDUP([Number of Days]/5,0)*2-2,
[Prepare Date]+[Number of Days]+ROUNDUP([Number of Days]/5,0)*2
)
)
References: WEEKDAY function. / ROUNDUP function.