I got this advice from a couple of people, one the Engineering VP at the last job [i.e.: 40hr/wk nominal, W2] I had, back in '79, one the CEO of the company before that (whom I consulted with for ca 20 years during the 80s, 90s, and 00s) after he started up his own consulting/contracting company.

Basically, take your desired salary[including expenses/fringes], divide by 100, and that's your daily rate. You can come down a little for long term work, say more than 2 weeks.