Basic ProbabilityThe first rule to assume is that an average for a d6 will be 3.5. By extension, you should expect an average of:
7 on 2d6
10.5 on 3d6
14 on 4d6
Common consensus is matching these numbers will be what you need. However, that's only half-true. Needing a 7 on 2 dice will only yield success 58% of the time which, while statistically likely, isn't all that likely (less than 2/3 chance).
This becomes even less likely, when you take in to account both hit and damage, in which case, it drops to around 1/3 success rate (assuming 7's for both). For instance your 10 attacks that are "likely" to hit and damage may only actually 2-4 enemies (accounting for some variation).
So, what you're actually shooting for is a little lower than the above averages: you want a 5-6 on 2 dice, which yield a much more satisfying 72-83% success rate. By the time you need 5's for hit and damage, you're looking at nearly a 70% success rate, which means that whatever survives to retaliate probably won't do much in response and, if you outnumber your opponent, you might be able to expect to take out a full unit.
"Long Shots"While that 11+ roll seems nearly impossible, it' actually perfectly reasonable to expect, with the right volume of attacks.
For the record, on 2d6, probability for…
11+ is 8%
10+ is 17%
9+ is 28%
So, while that unit might not look like its rifles can mess up a heavy jack, it might be able to do some actual damage- not a lot, but enough that it's not useless, especially if all you need is 3-4 points done. 12's, however, are not something to bank on, nor is it generally worth spending focus on a long shot, unless it's already assigned or the piece is going down next turn anyway.
An addendum to "Long Shots."
Something to consider is, what exactly constitutes a "long shot"? Because there are circumstances when things are easier than they look.
One I've started aggressively taking advantage of is firing in to melee:
Typically, the targets for this are knocked down models. With knocked down models, you'll typically only be trying to hit DEF 5, which is in the reach of even embarrassingly inaccurate attacks. With other targets, it can still be worth it of you've got the opportunity to do a couple points of damage. The second half of this is making sure that any friendly potential targets (if the attack misses) are on no real jeopardy or are able to be sacrificed. Warcasters, for instance, are typically well outside of a reasonable chance of hitting: the lowest DEF Warcaster would still be effective DEF 17, while a Heavy is typically going to shrug off small arms fire. Similarly, if your Light loses 3 health with a high roll, will that effect you? (Sometimes, the answer is "yes.")
Important RollsThere might be times when something needs to be taken out. Never save these rolls for last. Get these out of the way as early as possible (sometimes, things need to go before to buff, get out of the way, shoot before something else engages, etc.). That way, if and when something doesn't go according to plan, you have the rest of the turn to compensate. This has saved my game countless times.
Also, never, ever rely on averages for important rolls. Be ready to invest extra focus and extra attacks to get the job done: it's better to be inefficient and have a spare focus sitting on your heavy, than to be sitting around with nothing left to attack with, and an enemy heavy breathing down your neck.
Threat RangesWhile this is primarily a look at how to handle basic probability, I would be remiss to not mention threat ranges. This is something that you'll learn with time, but, even now, if I haven't played against a piece, I'll ask an opponent what threat range a given piece has before the game starts: there's no shame in not being a gaming encyclopedia.
There are a few different threat ranges:
1- basic threat range. Simply, movement + ranged weapon range, or movement + charge distance + melee reach. This number can mean very little, as it often isn't what will actually be coming at you.
2- running threat range. Running is an option to hamstring enemies, and not just ranged ones. Ranged enemies, of course, will be stopped from shooting if stuck in melee. However, a viable strategy with things that rely on charges (for ability boosts, or just a boosted damage roll) is getting within three inches of them, preferably tying a few up so they can't shuffle around and charge at funny angles.
3- potential threat range. This is either of the above, plus any bonuses from out of turn movement, spells, aiding solos, etc. This is the real number to be aware of. That SPD 4 model, with a charge and reach, has a threat range of 9", which becomes 13" with a couple more abilities slapped on, so suddenly that 4" movement doesn't mean much.
The concept of a threat range is just as relevant in other stats, that you should learn to be aware of. The same concept of basic vs. potential stat works in a number of contexts. Does your army rely on armor? Make sure you know how high that seemingly insignificant unit can push its power. Does your army have trouble with accuracy? Be sure to know how what DEF can be pushed to, especially if there's terrain that can move it higher. Etc. etc.
ConclusionThere are two extremes in wargaming (and preparation). On one end, you have the person who rolls with the punches and is satisfied with a general sense of the game and, barring great intuition or luck, will probably muddle through games. The other end know the rules of every current piece, can cite from memory most of the other ones, and publishes Excel sheets on dice statistics and probability, and their armies are cold, efficient, killing machines.
Neither of these extremes are my MO. I like to play and have a characterful army, but I also like to win. It doesn't take much to be ready for any situation. Come to a game with a decent army that you know well enough, and be ready to ask the important potential stats you'll be up against. In the immortal words of a late-80's cartoon which, coincidentally, also had a story designed to sell expensive toy soldiers, "Knowing is half the battle!"