I think it really comes down to personal preference and what you feel comfortable charging in the end. I started with a sq in formula to get an idea of prices just because it makes things more simple and straight forward. Fixes a starting point when you have no clue where to start. I've added over the years as skill has improved, upgraded materials etc, but I've just added onto that base price I started with and still do base it on size because it's just easier and more streamlined