Glenn:
The key here is that the liners are fabric and padding and other materials that will stretch and compact.? Where your toes are in the liners at this point in time is virtually irrelevant.? The thing that is really important is where your toes (and your heel and the rest of your foot) is in the shell itself, because that is plastic and pretty solid and won't give or change very much (except get much stiffer in the cold).
As you use your boots, the liner will shift and compress and expand as a reaction to the pressure exerted by the foot until it reaches the limits imposed by the shell itself.? If the shell is too big, the liner will very quickly be too big, and you will find yourself cranking the buckles down more and more trying to make the shell smaller (a virtual impossibility). As others have said, the basic guideline is 1/2" of space between your heel and the shell when the toes are just touching the front of the boot.? The other thing you are trying to estimate, as Ron/Gary mentioned, is the clearance from the side of your foot to the side of the boot. Same idea, you don't want the liner expanding too much side to side or front to back.
The good news.? Welcome to the world where we have all been at one time or another, buying a boot based on "feel" in the shop.
Rule #2.? You can make a tight boot bigger (with slight grinding away of plastic), but you can't make a big boot smaller.?