First and foremost, the numbers I quoted were based on consulting work. If anything, they may be down as I have been out of the consulting game for several years now.
But I would seriously question whether the OP should stay at the same rate. There is no doubt his costs will go up, just based on FICA and overhead alone, and the employer’s costs will go down (FICA costs and benefits). There is also a very good benefit for the employer using a long-time employee as there is no uncertainty that you get with any new employee. Even if the new employee is as good as or better than the OP, which there is no guarantee, there are always short-term training costs incurred.
I will also say the other terms of the contract matter. If the employer is guaranteeing a year of part-time employment, that only works well for the OP. Most consulting work is 2-6 weeks. Then you have to track down new work. A year-long job would have a lot of weight.
But if you had to nail me to a number, I’d try to get 1.5x current rate for a 1 year commitment. I would not take less than 1.3x. At that point, I would hussle other business. And of course, I would be hussling other business while I was working part time.