Some random things about boot flex in general. I'll use "you" but not to mean anyone in particular.
--Diagnosing whether your flex is too low
If you ski on warm days when the snow gets dense and wet, you'll find out real fast whether your boot flex is too soft. The snow will press back on the fronts of the skis, which will force the shins into the tongues. No problem, that's the way things work with skis and boots. But if the boot folds forward as this happens, then you are in danger of face-planting, and you'll be immediately aware of this. Boots should not fold forward so easily that the skier feels insecure about falling forward when the snow pushes back; if they do, the flex is too soft. ........All boots stiffen up in colder temps, but skiers ski in snow that changes continually. The boot should give you enough cuff support to keep your lower leg upright when the snow pushes back. It should go without saying that if the boot folds forward even in cold temps, then stiffer boots are needed. ........It's not only conditions reveal that boots are too soft. If you begin skiing faster with higher edge angles as your skills increase, an older boot may not offer enough support. If your skills increase and you start skiing cut-up crud, your old boots may not offer enough support. If you move from an area with champagne powder to the pacific northwest where the snow falls wet and dense, your old boots may need to be replaced with stiffer boots.
--Diagnosing if you are flexing the boot or just the tongue
Boot flex happens when the back spine of the boot bends forward. Stiffer boots with higher flex numbers have stiffer, thicker spines. If as you ski the spine doesn't bend forward but the tongue does, allowing your lower leg to tilt forward, that's not due to the boot flex. It's due to the fact that your lower leg moves forward in space, inside the cuff, away from the boot's spine. If you get a gap back behind your calf when you "flex" the boot, and it looks like you are moving the tongue forward but not bending the spine, then the boot is not flexing at all. The boot is not being used to its design specifications. Skiers buy Booster Straps and have them put onto the boot to replace the non-elastic power strap. The elastic Booster Strap binds the lower leg back against the spine securely so the skier can access the boot's flex potential.
--Diagnosing how high a boot flex your skiing requires
When the lower leg is firmly bound the the spine, and you bend forward at the ankle to hover your upper body's weight (including hips) forward over the front of the skis, the boot acts as a lever. It levers the front of the ski downward onto the snow. (You can stand in the boot in such a way that the tail stays pressed down while the tips are levered; that's another topic.) It's good to gain the skill of levering the front downward (aka "getting forward"), but too soft a boot flex can undermine the development of this skill. ..........A boot with 140+ flex will not flex at all unless the skier is moving with significant speed and edge angle. That boot's lack of flex maximizes the levering action, which is what the skier in that boot (often a racer) wants. A skier in a 140 boot will want immediate translation of body action to the ski and will be confident that their technique is precise and that no inadvertent body wobbles are going to happen. ..........On the other hand, an 80 flex boot's spine will flex easily, even if the skier is going slow with skis flat. A cautious beginner on low angle pitches learning to ski needs a softer boot that will flex when they make inadvertent body wobbles. The boots will absorb most of those movements as it flexes, little levering will happen, and the skier will be safe from surprises from the skis. The absorption from the low boot flex will blunt the effect of the skier's lack of control over shin tilt and where their body weight is relative to the ski's length. ........A boot with a flex somewhere in between 140 and 80 is what most recreational skiers need. How high you want to go depends on how much absorption you want before the tips get levered downward. And that depends on what kind of conditions you ski, how fast you want to ski, how high you are getting your edge angles, how much your weigh, and how precise your movements are. Choosing to go one or two steps higher in boot flex with each new boot you buy as your skiing improves is the way to go. Advice from seasoned skiers who have seen your skiing is often helpful.
--Altering a boot's flex
A boot cannot easily be made stiffer in flex. An after-market liner ($$) can maybe add some stiffness. .......A stiff boot can be temporarily softened by loosening the cuff or by removing one of the screws in the spine. It can permanently be made softer by a bootfitter cutting into the shell in various ways.
Hope that helps.