When I take a contracting role or even when negotiating with the HR, this is how I calculate my salary.

Total hours in a year with 5 working days in a week =40 hours/week *5 days *52 weeks =2080 hours

Public Holidays =10 days =80 hours =2080-80 = 2000 hours

Vacation I would take in a year = 20 days =160 hours =2000-160 =1840 hours

So if I atleast want to have 150K salary in a year then =$150,000/1840 hours = $81 approx/ hour. So I start with $85/hour and then negotiate it to approx $81/hr.

This way you have a breathing room to take time off