Duane, since you say you already know your salary range for regular W2 employment, I agree that a formula can be applied to ascertain a 1099 equivalent. If you do a search for the total cost of an employee, you'll get a figure somewhere in the neighborhood of 18-26% of the base salary for the cost of benefits.
You should also take into account the fact that you do not get paid vacation, holiday, and sick time. To make it easy, let's say an employer should expect that a salaried employee would get 4 weeks out of the entire year for vacation / holiday / sick time (out of 52). 4 is about 7.7% of 52.
Also think about what equipment and services you will need to provide yourself. If they do not provide a reasonable work space for you, factor in how much such a space will cost you. If they will not provide your work machine, factor that in as well. If they require you to have an internet connection and don't provide it, add that. If they require you to call in but don't provide you with a phone and/or don't pay for yours, add in some percentage of those services.
So as a baseline, you should be looking for around 1.25 to 1.5 times the base W2 salary plus the cost of whatever they will not provide you in order for you to do your work for them. Divide that number by 2,080 (the number work hours in a year if you work 40 hours a week), and you'll get a bare minimum reasonable hourly rate if it is a full time gig. If it is less than full-time, I'd tack on at least another 25% (if not more).
If it is a full time gig, then the employer will likely expect a 'volume discount' such that they would not spend more on you than they would a regular salaried employee (approximately the above formula). However, if this is a part-time gig, you really need to charge more per hour than the above calculation. If you only work for them part-time, then I would treat yourself more as a company providing services to a client. You must account for your overhead and expenses as a company - for example, the cost of finding other work, etc.
There are other less tangible aspects of contracting vs. employment. There are usually 3 reasons why the employer might opt for this arrangement:
1) It is contract-to-hire, meaning they want to test you out before fully committing;
2) They don't know how long they need you or know they will only need you for a short time, and therefore it doesn't make sense to bring you on board as an employee;
3) They aren't organized or big enough to deal with having an actual employee.
I would try to figure out which of these (or another I haven't thought of) is the case and price / plan accordingly.
I own a contracting company that charges an hourly rate that is about double the regular W2 salary of someone that does the kinds of work I do annually. However, this accounts for the fact that I only ever work for another company part-time and have overhead of equipment, utilities, and the cost of managing the business, and the cost of finding new business to fill the rest of my time. If yours is a full-time gig, I doubt that the employer will go for double the salaried amount, but 1.5+ may be reasonable.