Help with Boot Fit for Ice Climbing

So, I've bought a pair of Mont Blanc GTXs, and I could use some help in determining how much room I need in the length. The heels fit very well. Surprisingly well, actually. virtually no lift.
I'm wondering how hard I need to kick to check whether there is enough room.

If I kick quite hard I can get my toe to hit the front of the boot in about 4-5 kicks.

 I am of course concerned that if I size up the heel will get loose.

Have I found the sweet spot; or should I not be able to hit the front under any circumstances?

First one ever with this dilemma, right?

It's hard to help with this kind of thing online, but you're going to kick hard hundreds of times per day. Walking downhill might suck too. Try on the next size back-to-back. 

With the boot totally unlaced, and your foot slid forward as far as possible, can you slide a finger behind your heel? That's always been my standard for boots of all kinds. If you've got that room, a tightly laced boot shouldn't allow your foot to slide so much that your toes hit the front.

Sounds like they don't fit well.

I'll disagree with everyone else and say these sound like they fit well for ice climbing. The thing is, you're not kicking the ice with the boot, you're kicking it with the crampon frontpoints (if you're doing it right). Kicking a stair directly will drive your toes to the front more than ice climbing will. Ideally, you can try the next size up and see if they hold your heel properly but if they are a looser heel fit, I'd stick with the smaller size. Sometimes the larger sized boot will let your foot slide around more, making the toe bang problem worse. All this will also depend of how much walking vs. climbing you'll be doing in the boots. I would want a different fit for Rainier vs. waterfall ice cragging.

If it was me I’d try to size up. If you still get decent heel fit there would be no reason not to.

Feet can swell over the course of a day. Long descents can really kill in a slightly tight boot. When you’re pumped/panicked you might really kick the crap out of some ice.

I’ve got more than one pair of boots that fit great most of the time and then occasionally are just too dang small. Can’t really explain it but I have learned to size up a bit from what initially feels ok. 

^^
Agree 100%.

Err on the side of too big for boots. Opposite of rock shoes. I've had to resell a few pairs over the years. 

Hi everyone,
thanks for the fine suggestions. G'Mike's finger trick is especially helpful.  I have more than a finger's space behind the heel. But I have ordered the next size--as suggested, it seems very well worth the cost of return postage. Why can't these things be easy? First World problems, I guess, so I shouldn't complain.

Thanks for the help folks. Be Sweet.