You can still calculate an ARV whether it's a flip or not. First off, I ALWAYS factor in the square feet of the house because you can have a 3 bedroom, 2 bath house that is 1,200 SF and a 3/2 that is 2,200SF. I probably spend too much time calculating ARV but it's the accountant in me. I find recent sales in the same neighborhood as my subject property because in NJ, prices can fluctuate a block or 2 over. Just make sure your comps are no longer than 1 mile from your subject property. I then find properties that have sold preferably in the last 6 months but no more than 12 months ago. Whether I come up with 10 homes from that quick search or 20 homes, I then calculate an average $$ per square foot just to know that figure.
Now try to find as many similarities in the subject property as in the 10-20 homes. I have a spreadsheet that has columns for everything.....BRs, baths, Finished basement, finished attic, granite or formica countertops, hardwood or laminate, vinyl siding or aluminum siding, two-car wide driveway vs single wide, etc, etc. Compare those columns and whittle down your houses to 3 or 4.
Obviously some things are subjective like quality of finishes but the more you do it, the better you will get at it. I like to compare the $ per SF of those 3 or 4 versus the average I came up with for the 10 or 20 homes. The average on the 3 or 4 should be higher because you're looking for remodeled homes but you just want to make sure the difference in averages is not drastic. You now have a fairly accurate $/SF for a remodeled home.
Hope I explained that ok. Reach out with any questions.