I can answer from my own experience. And that would be to say, the ideal leg opening size is certainly not universal. It depends on all kinds of factors: Your body type; the width of your calves and thighs; the rise of the pant; your height; the kind of shoe you intend to wear (heel or no heel, round toe or pointy); the overall style of the pant; your purposes (i.e. what kinds of outfits you want to make); the level of looseness or snugness you generally prefer.
The width of the leg opening itself doesn't even completely tell you about the degree of taper (in a BF jean, say) -- because that will also depend on the width higher up. Though it will tell you approximately how much it will stand out (or stick to) your calf, if you know your calf width. Assuming the measurement they give is accurate.
Can you tell I've done an extensive study of this??? LOL.
There's a reason. I am on the shorter side of regular height (or the taller side of petite, if you prefer). And I don't like "calf sucking" skinnies. So I have looked and looked for the leg opening size I need to create a skinny that doesn't cleave to my (relatively slim, yet still muscular) calf with the grip of the Grim Reaper trying to haul me to an early grave.
When it comes to boot cuts, I'll bet you can experiment a bit. If the ones you have are too wide, can they be tapered a bit to the width that feels best to you? Then you'll know your sweet spot.
I figured out my skinnies sweet spot (or area) by measuring all the jeans I have and comparing. Same with rises. For BFs, I prefer some taper but I'm not sure the specific leg opening size is as crucial as long as they can be rolled up (i.e. it can't be so tight that it can't roll up my calf.) But there's also the "straight loose stovepipe" type of BF jean that can look great on some; I haven't actually tried any like that so I'm not sure what the best opening would be on that for me.
This probably hasn't been a jot of help, but at least you know you are not alone in your obsession.