Rangefinding in HFT is a huge subject so I won't go into great detail since you're new to the sport and I wouldn't want to scare you off..
Since you've asked about bracketing though, I can tell you that on a standard Nockover target (the most common target used in HFT and FT) the distance from the top of the hinge plate to the centre of the killzone is 110mm (except a 15mm Squirrel target which is about 112mm). You can also take measurements of each different target type and produce a range card from that.
But.... this isn't a particularly reliable method for lots of reasons - firstly, taking precise measurements in mildots isn't easy. It's extremely easy to make a mistake on your reading which will put you miles off on distance. Secondly, this is a well known method by course designers who will deliberately throw in a curve ball target to really mess with your head.
It can be a useful method if used as a last resort, but shouldn't be used as a primary method of range finding.
There are a number of methods however... including the good ole Mk1 eyeball (which also shouldn't be totally relied on either because course setters will also place targets to fool those that range by eye).
I'll briefly list some of the various methods...
1. Learn the rules of course setting - if you see a 25mm killzone, knowing the rules will tell you that it cannot be any further than 35 yards for example. Similarly 15mm kills will be between 13yards and 25 yards.
2. Learn from previous targets - once you've shot a target and seen where your pellet landed, that should give you a very good clue as to the exact distance to that target. You can then use that target as a reference for other targets on the course. If a target was 40 yards on a large tree for example and the next lane has a target that's a bit further than the tree then you'll know it's over 40 yards etc.
3. Look for reference points on the course - such as targets placed on or near fencing with equally spaced posts (as most are). You can't pace the distance to a target but you can pace the distance between two fence posts behind the firing line and then multiply that by the number of posts between the line and the target - there are other examples of this which I won't go into, but you get the idea - keep your brain engaged and look for clues and reference points.
4. Bracketting - you can bracket killzones, hinge to centre of kill, full faceplates, partial faceplates etc etc.. this can be a handy method as a last resort or to double check the range, but use it sparingly and don't trust it if a more reliable method is available.
5. Ranging by eye (be aware that targets in a tunnel of trees will look further away, targets where there's dead ground between the target and the line will also look further away).. There are lots of ways that rangefinding by eye can be fooled, experience will help you to learn when this is likely to happen.
You can practice this method while walking down the street etc.. try to judge the distance to a lamp post for example and then pace it out to see if you were right or wrong etc.. try not to do this in a crowded area though or you'll get some strange looks
6. Rangefinding by parallax - as you mentioned, you will notice that at certain ranges the image of the target and the crosshairs will both be in very sharp focus. The MTC viper has quite a poor depth of field so although you say it's "pretty" clear between 8 and 45 yards, with practice you'll learn the difference between pretty clear and crystal clear and be able to use that to help with rangefinding. You can practice this by first concentrating on the traget then on the crosshairs.. if you notice that your eyes have to re-focus when switching between the target and crosshairs then you'll learn how to tell the difference between sharp focus and blurred focus. It does take practice, but this is the most reliable method of rangefinding in HFT (and it's the method used to rangefind in FT as well).
Another way you can use this method is - suppose your scope is in sharp focus at 23 yards and you are able to get that distance about right within a couple of yards.. you can then use that focus range to look around for things on the course such as trees etc that are at exactly 23 yards - this will give you a good reference point that can then be used to judge the distance to the target.
Something else for you to consider... You don't have to hit the target in the centre of the killzone for it to go down, so if you're wrong by a few yards it doesn't matter.. instead of trying to come up with a single distance for a target try to come up with a range of ranges so to speak... for example you might think that a target is between 30 and 37 yards... have a look at your aimpoint for 30 and your aimpoint for 37... can you get both aimpoints inside the kill? - if so hedge your bets and shoot the target for 33.5 yards.
I said I wouldn't go into great detail didn't I... Ooops..