Updated: Sep 13, 2020
Sometimes I see Flex Boots where the back strap has been adjusted too tight. The tight back strap does not hold the boot any better on the foot than if the strap is adjusted firmly. Actually, if the back strap is too tight, you may loose the boot because of that.
You should be able to add one finger under the strap when properly adjusted, so the heel bulbs have room to expand in every step.
Above is a photo of our young horse. He wears Flex Boots 24/7 now that it is slippery again. You can see that there is space between the hair and the strap under the neoprene gaiter. Actually, the strap is little loose, but when the boot fits well, and the trim is good and done weekly, the boot stays on perfectly fine. If the boot is slightly loose, FlexPad EVA or FlexPad KEVLAR offer extra protection to the sole, stimulation to the frog and sole, and helps to keep the boot in place.
Here are two examples where the straps are too tight.
The holes on the straps are clearly stretched, and it should not happen.
In comparison, below are photos where the strap tightness is perfect, and there are no stretched holes. However, in these pictures there are two holes underneath the rivets, but it's actually recommended to cut the strap shorter, to leave only one hole beyond the rivet.
Once you get the strap tightness right, if there is more than one hole underneath the rivets, you can cut the strap to have only one hole left, as shown in the photo below.