You would need to calculate the total hours which you expect to be a shrink. For example, 3% and 1.5% that is close to 21.6 minutes. Now you would need to add the shrinkage as per the absenteeism trends at your centre or any other shrinkage applicable.
To calculate the actual requirement, just divide the base requirement by 1-shrinkage and it will give you the actual requirement. i am assuming the weekly off shrinkage has already been considered in base requirement.