This is how I found perfect size of ski boots for myself :
My everyday walking shoe is size 12.
In the ski store the device for measuring the foot
size shows #11,which corresponds to Mondo #29.
I chose ski boot size 27.5, Yes 27.5!!!! to get the right fit
for advanced level agressive skiing.(I used to ski at professional
level in Poland,and took part in many competitions)
When I stand up straight toe tips slightly touching
the front of the shoe,when I lean forward they are not touching
at all. My foot is narrow,I chose 99mm last (shoe width).
I like the boot to fit snug with 0 play, so I can have 100%
control over my skies.Most people buy ski boots too large.
My advice,when you try new par of ski boots on ,and they feel comfy like
a slipper,DO NOT BUY IT.I use Fischer Soma RC4 Comp 110 and 2010
Fischer Progressor 9+ skis 175cm (I am 6'1" tall),nice set believe me.
Hope it helps because it is not possible to give a universal advice
for everybody is different.