I know the OP's already headed to boot fitting, but I think I take a less traditional approach. I have an amazing fitter who I trust dearly, but I start by trying all the stock boots on. I look online and ask friends about which shops have which boots and I try to get my feet in a bunch of sizes. (I measure a 25.5, usually wear a 23.5, but I'll test out everything down to a 22.5 just in case). I try them all buckled up, but if they're a touring boot, I'll also try to explore the ROM with stairs or lunges. I go into my fitting with a list of boots and notes, and know which boots have the most potential and which ones would need which modifications.
We usually agree on something that's a little too tight out of the box, so I'll wear them around the house, begging the liners to pack out. Sometimes that's enough. Others, I'll keep a marker in my car and circle the painful spots on my feet after a tour to inform where we should punch. I have an insanely low instep, and as a former ballerina, my toes have been conditioned to a lot of abuse and can handle tightness and pinching. It's not uncommon for me to leave the store with something a little too tight to accommodate my volume. This past season, I went all winter happily skiing a pair of Zero Gs, but realized on the longer 10-14 hour days that my toes were unhappy.
Can't +1 the recommendations about going on less busy times of the season or day enough. Totally helps to get undivided attention. And I also push friends to leave appointments where they don't feel heard or trusted. There are lots of talented bootfitters out there, but they won't work magic for you if they don't communicate well. I'd rather go to a fitter with fewer accolades who wants to work together on my bootwork as a team than one who wants to minimize my participation in the process.