Personally i have two rates. One for regular and trade clients and one for standard non discounted work. £35 per hour and £45 per hour.
You would then discount your rates for a days full work or a weeks full work etc.
Ive been freelance for 6 years though.

To calculate your rates is quite common sense.
Figure out what level of experience you have and work out what you would be paid or would like to be paid if you were at an agency.
You then need to consider that as a freelancer you may not have enough work for a third or up to half the year.
Divide your expected salary by 26 weeks and then that figure by 5 days and you have a daily rate to aim for.
eg £30000/26 weeks = £1153/5days = £230 per day or £28.80 per hour