Why add holidays and sick leave to an individual employee's costs? Most white collar employees would have an annual salary this is factored into already, unless you're having to hire temporary replacements for them perhaps (which would be more than a 15% cost, I suspect).
You are right, I usually do this to get to a worked daily or worked hourly cost (I work in consultancy) so I take holidays & sick into account to understand cost per worked hour / cost per worked day.
In reality it depends on the type of work you are doing.
Statutory allowances in the UK are higher than in the US, and in the UK for instance you would add:
- 12.5% national insurance + apprentice levy
- 3-4% pension
- 10% of days as holidays
- 5% of days as sick / other leave (dependent on amount taken in reality)
- 2% payroll fees / other
Note, this excludes training and equipment/office space.