I’m your stats twin. I went from 171-165 doing a 3.5 day fast (but I drank coffee with creamer and bone broth/fish stock).
I had a co-worker do on a 21 day water fast (not for weight loss, but other health related issues) and she dropped over 20lbs (she’s 5’8” and maybe 150lbs).
If you want to just count fast alone and not water or food weight, then perhaps a little less than .5 lb/day (just take what your caloric burn is that day). However if your body gets excited that it has free access to your fat stores, it might rev up and burn lots and lots of fat (more calories) than it normally would! But I would lean on the low side so then no matter what you’re excited about whatever happens as a bonus!