I'm from the UK, so it may be different to wherever you are, but as has been stated, I don't think caterers are paid with tips in mind like some waitstaff (a practice I have never been keen on), so there's no need to necessarily do it mathematically.
I worked catering for a long time, and staff tips were extremely rare, and appreciated whenever they were received - not many people ever tipped the staff, and this was a high end catering company. It just wasn't really expected. Once we were allowed to take home a bottle of the leftover wine/champagne - that was nice at the end of an 12 hour shift.
Again, practices may be totally different, but I think a small amount per staff member would be appreciated - 15 or 20 perhaps, and I can't imagine anyone would compare it to the bill.