Here's another short video on boot flex. This is Reilly McGlashan. He's an elite skier who explains why one might want a stiffer boot, rather than a softer one. He skis very aggressively, and with enviable skill.
He suggests that a skier should not shy away from buying a stiffer than expected boot. He mentions that some skiers do find themselves skiing back-seat when they are in stiff boots, but explains that the boot is not causing this. The boot does not "push" them into the back seat. He says these skiers will find the stiffness of the tongue-shin contact to be intimidating. So they pull their shins back from that contact and this immediately puts them in the back seat. If they kept their shin-tongue contact, they would not be skiing backseat.
He explains the benefits offered by a stiffer flex boot, and he lists what kind of snow situations are best skied with a stiffer boot. He also notes when one might want a softer boot, and how to temporarily alter a stiffer boot for those situations. It is important to know that a stiff boot can be temporarily made softer, but a soft boot cannot be made stiffer.
He is talking to an audience of technically ambitious skiers, and you
@AJM sound like you are one of those. But given the flex numbers he discusses at the end, I think he assumes he's talking to technically ambitious skiers who want to ski aggressively all day. Not many ambitious intermediate and advanced skiers are ever going to want boots with a 150 flex, whether female or male, petite or big. Most skiers won't even want 130, but it works for some ... and 110 definitely is fine.
Implied in what he says is that skiing boots with flex numbers below that probably will limit the skier's ability to vary their control over their turns as they climb the skill ladder.