Most often people will use hourly pay rate times three to determine a general freelance rate. But that's merely a ballpark. There's no "one method" which is accurate for everyone. Your general overhead as a contractor is higher because employers aren't paying for many things. In some cases basic necessities to work such as electrify and equipment. So typically a freelance rate is 3x what a current hourly rate may be as a general guide.
If you are not interested in covering overhead - e.g. electricity, water, equipment, supplies, etc - because you will be working on site. Then you need to factor in your current employer contributions. If the employer is matching 401k, that may stop. You won't have paid vacation, sick, or personal days. If the employer is covering health insurance, that will stop. Health insurance is a BIG one because it's going to cost you way more as a contractor than it does an an "employee". I still would lean towards the hourly x 3 though. But if you like the employer you could possibly do hourly x 2 since they will still be paying for general overhead such as equipment and electricity.
Be aware, may employers try and put workers into a 1099 arrangement so they can avoid things like health insurance and workman's compensation. The IRS started cracking down on this behavior, but that does not mean it has stopped.
If an employer tells you when to work, what to work on, provides tools for work... then they are an employer not a 1099 freelancer/contractor client. I realize that may not matter a whole lot to you, but you should be aware that if the employer is attempting to put you in a position better for them, but worse for you, you may just want to find a new job.