I'm not historical, but generally paint custom homes. By "custom", I mean mass produced, over priced homes built on former farmland.I'd have a real problem with sq.ft. pricing because so much of what I do is historical homes and each one is different. When I see the job I can also see what complexities might be involved. A price per sq.ft. on one house will be different for the next & new builds would be cheapest of all because they tend to be simple and straight forward. Nope, couldn't do it that way. I look, assess and figure out my time on site.
I spend at least 20 minutes at each home just wandering around, looking for problems. It also helps me point out other things that may need to be addressed, so the price doesn't shock them.
I work out my general number with an equation, then figure out extra hours needed for spackle and fixing the inevitable, horrendous caulking, deplorable mitre cuts and gouges on new trim. I also add in a bit more when the final product they want requires a certain amount of skills that may not be universal.
I have to say, doing historic houses puts you leagues beyond me. The HOs for old homes I have done tend to be of the "make it look pretty" ilk.
I've had to turn down jobs on major restorations. Brushing all of the walls is NOT me! Staining terrifies me, unless it is a deck! Add in lath and I cry!
Sincere props. Despite my fears, that is where I want to eventually be.