I am not an accountant or anything but I think a rule of thumb is employment costs 50% more than the employees salary. Don't feel too bad asking significantly more, after all you are not getting any equity which sounds like your main motivation.
That sort of rule of thumb is going to be most accurate for average salaries, because large amounts of the non-salary costs are relatively fixed (e.g. health insurance costs the same amount whether you earn $50K or $200K annually). As salary goes up, so does its share of total compensation.