What does the right amount of flex supposed to feel/look like when geting new boots?

[Zirbl]

"The point where I feel bellowing of the lower shell happens when the ski is loaded"
Is the clog too big for your foot?

That would be a fair assumption, but no it's not.

 

[KingGrump]

01 Around where the cuff and the clog meet. Probably starts feeling mush around the apex.
02 Referring to the pressuring phase starting around the rise line and ending whenever it's time to release the ski.
03 Actively closing the ankle joint, staying low through transition. (So not trying to smash down on the top of the cuff, if that's what you're wondering.)

The apex is usually where the max pressure is introduced to the forward flex of the boot. When you say where the cuff meets the clog, I am assuming that location is at the front of the boot. Basically the around the instep. If the bellowing is occurring at that spot, you are probably experiencing the result of interference between the cuff and clog. Start Haus has a good video on fixing that issue. That fix starts around 7:30.

Most obsessed about getting to front of the boot. Yet most of the power/performance of a ski boot lies in the spine. How do we get to the spine? That is the key.

The first part of that video is on the proper way to soften the flex on a boot. It should give you a good idea the location of the bellowing when the boots are too soft.
The third part of video covers work on the top of the cuff. Fine tuning the boot to both the skier and skiing style with regard to the placement of the Booster strap.

The shaping phase start at the rise line. that is when the pressure starts to build. Usually peaking at the apex. To me transition is usually a very brief moment in time. The time when the skis are flat. Once the feet start to roll up on on edge (eversion/inversion), I considered that as the top of the next turn.

 

[Zirbl]

Thanks @KingGrump , that's very helpful.