How tight do you fit your shells?

I'm swimming in a pair of $$$ boots I purchased at a reputable local shop last year. Time for more boot shopping. Curious how tight people fit their boot shells for primarily backcountry use. Internet searches say anything from 3/8" room to 2+ fingers. I good friend goes down two full shell sizes from their street shoe. Seems tight! Any input much appreciated.

I would say if your toe is touching the front of the boot, then 2 finger gap in the heel is about right for a touring boot, less for alpine. Keep in mind this is only for length of the boot, not for total volume of your foot/particular shape of the last used to mold the boot. Personally my street shoes are also much larger than my ski boots, or any hiking boots/shoes that I wear, so two sizes off could be right depending on how large you wear your street shoes.

Tight is good in a ski boot. Most people will say snug because tight has negative connotations - I don't mean suffocating/cutting off blood flow. If your foot has room to move around, it will. In downhill skiing, movement = jamming your toes into the end of the boot = pain. Movement in the boot also means less control as your bodies inputs aren't being directly transferred to the boots. For touring (and most aerobic activity involving your feet), your feet will probably swell a bit - and since you're spending 90%+ of your time going uphill, it makes sense to size up to a 2 finger gap or larger depending on your goals (uphill vs downhill performance and comfort).

I would talk to whoever at that shop put you in that boot.. and/or go to a different shop

I've always gone by the rule of standing in the shell with a thin liner sock, toes just touching the front, and being able to just barely fit my index and middle fingers (stacked one over the other, which is approx 25mm) behind my heel and the shell.  I wouldn't consider this a 'performance fit', but its worked well for multi-day trips, long climbs/approaches, etc.  My resort skiing boots are a single finger fit in the shell.  

I think using fingers as a rule of thumb works best if you are doing the measuring with your own hand... if a friend or shop employee (who doesn't have much experience)  has small or large hands, then the whole thing could be way off.   

Too bad the shop got you in the wrong size boots... I just recently got a pair of boots and waffled on whether to get them online or from a local shop- was very glad in the end I went local- they got me into a shell size that I would've initially thought was going to be too small but after a couple tours and a couple shell punches, they fit like a charm.

Shell fit- I have been taught that the fit should be between 1-2cm behind the heel with the toes touching the front of the shell. It's also true that a good boot fitter can make your boot bigger and provide more room, but making it smaller is hard to do.

That said, I spend waaay more time climbing up in my BC boots than I ever do skiing down, I don't prefer a "tight race like" fit for AT.  My street shoe is 8.5-9 (and my feet seem to be spreading still) and I'm in a 26 for AT (2.5cm behind heel), and wavering between a 25.5 (1.5) and a 24.5 (<1cm) for inbounds. That 24.5 is going to need some work in the toe box but the heel hold is amazing.

Shell width is just as important for me as length. Having a narrow foot means that I will never get a precise fit out of some boots while others (TLT 5 and 6) fit great.

I would concur different boots definitely fit differently regardless of size. I just bought some Maestrale RS's and they fit very snug even though I have a narrow foot and most people say they are higher volume. My toes feel pretty constricted but I haven't skied them yet and anticipate them packing out to be perfect. Its weird too because a lot of people downsize way more than I did in this boot.