A parallel occurs to me ...
Just as I have great difficulty taking a picture that has just ONE code violation in it, it seems that 'clever' folks practice their hustle in several directions at once.
While a 'square foot price' might have some role in analyzing a job, it (just like your 'hourly shop rate') has absolutely no role in preparing an estimate.
Why? Because then the focus is on the rate, rather than what needs to be done. It reduces you to a commodity - a path that leads directly to the trunk of the local, unlicensed, 'handy' sort.
Just as the old DI used to harrass recruits by asking them 'how long is a rope?', asking 'how much per square foot' is meaningless without first knowing what the job involves. Without a plan, that's a wild guess at best.
Oh, I get it ... YOU are supposed to spend your time making the plan ... share it with the customer ... and take a chance. How about the customer first paying for your design work? Mention that idea; I bet their look will be priceless.
