Sorry to give such a useless answer, but it is the correct one.
How many years of experience do you have? How good are you? How much value to you bring to the company? How much profit have you directly brought the company through your work? How big is the company?
I’d do it this way…
Based on all of those answers above.
Decide what your full-time salary should be, or what you want it to be.
That # is based off of a 40-hour week. Now reduce that by half for 20 hours a week, tweak accordingly if you want to do 24 hours per week.
Will your boss/client be taking taxes out? Factor that in if necessary.
Now, since your boss/client won’t be paying benefits, vacation, or anything else, I’d feel comfortable adding in an extra 10% of that final total. This part is entirely up to you. That is likely what I would do. The issue is, you’re asking for this part-time role, so your employer thinks they’re already doing you a favor. They don’t necessarily “owe” you the compensation for vacation and benefits, as you’d simply become a part-time freelance consultant, not an “employee”… but for your own self-preservation, I’d factor it in. But, it all depends.
This method isn’t the “right” way. Just a suggestion based on my experience/morals. YMMV.
