So, I’ve attached 6(!) layers of 2mm neoprene to cover the gap in the last picture. Made a big difference!
Now on the the top of the cuff. In the video I’m first moving without any weight put into the boot, then putting some weight into it. Seems like way to much movement. Should I: (a): stuff the gaps with more neoprene or (b): get a stiffer boot. It’s a Lange 110.
I agree with everyone saying your cuffs aren't fitting your lower leg properly.
It looks like your shin is pressing on the top of the tongue hard, flexing the boot like crazy, and to get that effect you are moving more than is normally called for. Your shin is flexing the boot, but a big part of your lower leg's movement is just getting the shin forward enough to touch the tongue, so that's wasted movement. The fix is to tighten the cuff a lot more than you have it tightened in the video.
The thing that flexes in a boot is the back of the cuff - the spine - that goes up the back of the boot. The boot's stiffness is built into the spine's plastic. So your lower leg needs to be bound to the spine snugly so it doesn't move forward without pulling on that spine. Flexing the boot is not about pushing the tongue forward, it's about pulling the spine forward.
The advice to buy a Booster Strap has to do with this issue of binding the lower leg to the spine. A Booster Strap is elastic. It is screwed into the back of the cuff, up there onto the plastic at the top of the spine. It wraps around your liner, clinching it close to your leg. Do not wrap it around the plastic flaps of the cuff, wrap it around the liner inside the plastic of the cuff. Then close and buckle the boot flaps around it. The Booster Strap does its job better the closer it is to your leg. The elastic allows you to tighten it solidly around your leg so that there is no gap in the back of your cuff when your lower leg tilts forward to pull on the spine. The non-elastic power strap that comes with most boots doesn't do as good a job of binding the leg to the spine.
However, If the boot's "volume" doesn't match your anatomy, this won't be good enough. You may need a different boot.
Volume means how much room is there in the shell right in front of the bottom of your shin and along the top of your foot where the cuff meets the clog (the lower section of the boot that goes around your foot). If you have thin lower legs and a low instep (do you?), you need a low volume boot. If you do, is your boot labeled as a low volume boot? It will have LV on the box.
If your foot/ankle/lower leg area requires a low volume boot, and you have a high volume boot, then no matter how tightly you buckle the two cuff buckles you won't be able to get the whole cuff to snugly surround the bottom of your lower leg. Your upper shin will be doing the work of pulling on the spine all by itself, and it will be banging into the top of the cuff, absorbing all the pressure you apply to the front of the cuff as a by-product of pulling on the spine. The bottom of your shin will barely make contact with the front of the cuff, and it won't be able to help pull the spine forward. You will get shin-bang, a painful bruise up where your shin punches into the top of the cuff.
Additionally, if you have a low volume foot and use a high volume boot, your ankle area will be narrow but the clog down there will be wide. Your heel and ankle area will move left-right inside the boot. You will be losing important rotational control over the boot and the ski attached to it. You may also have too much room above your instep, which will reduce your ability to tilt the boot and thus tilt/tip the ski.
Boot volume much match foot volume for the boot to do its job. Stuffing shims in there produces a poor substitute for a boot that fits. Your video does not indicate if this is part of your issue. Your boot may match your foot's volume, in which case all you need to do is buckle it properly and maybe buy a Booster Strap and use it around the liner. I hope this is the case.
Best of luck getting this sorted. Boot fitting is complicated and something many of us have had issues with. Getting it right produces big breakthroughs in one's skiing, so it's worth the trouble of seeking out a good bootfitter who will work with you to get the best fit possible.
