Advice on charging for photo editing work

Hi, all--I'm looking for advice on what to charge for editing photos.

I recently edited/improved photos to be used in a calendar for a non-profit organization (did that work pro bono).

A friend saw the finished calendar and would like for me to do the same for a calendar promoting her farm and business. She has offered to pay for my services.

Does anyone have input on hourly rates for photo editing? I'm not looking to "rake in the bucks," as I still consider myself at an amateur level, and this work would be for a friend. I'd like to hear others' input on a reasonable rate--even if you provided a professional rate, I could use that as a guide for what I would consider charging.

I have LRC, PS (though I only know how to do a few functions in PS), and Topaz AI (most recent version).

Thanks!