Giving out a flat +hit expected value is hard, because it doesn't entirely depend on you.
In this answer, I'm going to detail the expected bonuses an optimized player might have at level 11, what an unoptimized player might have at the same level, and lastly what your possible contribution margin to this gap is and how to solve it.
I have an optimizer in my party. We're at level 8 now and he sits on a reliable +19 to hit, I'm basing this section on his build, bringing it up to 11 (haha), but I guess similar builds are possible.
Please note that the vast majority of characters need to hit their enemies to deal damage, to debuff or even to buff allies. Increasing your chances to hit is a mantra of D&D 4e and every optimization includes raising the attack bonus.
This means optimizers -will- get these figures.
- +5 (half level)
- +6 (attack stat, starting at 17 or 18 plus racial bonus and increased at 4th, 8th and 11th)
- +3 weapon proficiency
- +1 class feature (on some classes. Some others get this bonus on weapons that only get a +2 proficiency)
- +2 expertise feat
- +2 charging (with the appropriate feat, only viable for weapon attacks)
- +2 getting combat advantage (which starts being really easy to get at level 11 with just a feat and a specific weapon, but is quite easy even before given the vast number of feats and powers that grant it, plus flanking).
You then need to add the magic weapon or implement. At this point in the game, the players should have a +3 weapon/implement (every weapon, amulet, armor or implement from level 11 to level 16 is a +3 one and you start getting level 11 items in level 8 treasure parcels IIRC, while you won't get a +4 item until level 13 or so).
Supposing they run on a +3, this totals to a whopping +24 (+19 for implement users), which is often enough to hit on very low rolls. Someone in the party might even be a buffer and add some power bonuses, for even more oomph.
- Well, you got a +5 from levels...
- ...but you tought starting at a 18 including racials was good enough, because you're not the striker, or because you actually cared about your defenses, so you sit on a +5 now.
- You use an axe, because axes are cool, and who cares about just one point of difference, so you get a +2...
- ...even if your class isn't really good with axes, or with anything if this matters.
- you do have the expertise feat, because they're awesome...
- ...but you're not a charger (or another build that gives you more bonuses)...
- ...and sometimes you do fish for combat advantage, because at this point you're desperate enough to keep up with your fellow partymen.
So this character, again with the expected +3 weapon, is trying to hit things with a... +14. around 25% less.
Your faults, if any
As you can see, having players build to optimize or not is a huge dial on the game becoming easy or hard, even more so than giving just some players a +6 weapon (that's a 15% by comparison).
Now, I get you're letting new players start at a lower level, and this might be another big problem. Every single level of difference counts so much in this game! With +1 to hit every 2 levels, +1 (from your weapon) every 5, +1 (from extra ability points) every 4, you're working with an average 7% lower chance to hit circa.
If you add their lower hit points and defenses, this means they're not able to contribute.
4e's assumption is that new players start at the same level as their peers and with appropriate gear. They were somewhere else in this world doing great things themselves, before joining your storyline.
What do I do now?
- First of all, be suere everyone has around +3 weapons. If you gave them better, all I can think is to raise enemies' defenses by the difference between 3 and their weapon bonuses, until they should have got the weapons they wield.
- Get everyone to the same level and to the same magic bonus.
- If there are optimizers, have them assist the other players in building characters who are satisfactory to play at their side. In my group the level 8 druid gained a +4 to hit in the process and now is more happily contributing to each fight.
If you don't want to step up to the challenge of playing against an optimized party, I suppose you might ask them to tone down their builds. Since toning down is arbitrary, I'd ask them to aim for 3 or 4 less attack bonus than they would be able to build for, and I'd expect other players to make an effort to be on par with them.
But, what about damage, Zachiel?
Damage is a completely different beast. Characters can be focused on being great defenders or leaders who deal almost no damage, or they can be basic attack beasts with items, feats and builds devoted to stack lots of damage.
The aforementioned charger deals +2 damage with basic attacks, wields a d8 high crit rapier, has a d8 sneak attack, deals +1d6 +2 damage when charging and +2 damage against bloodied enemies, and I'm surely missing 2d8 more from some class feature. He deals a bucket of damage every time he hits, often bloodying enemies his level in a single hit (around 50 damage by level 8).
Optimizing for damage is not necessary if you don't play a striker. If you do, I guess asking help to optimizers is the way to go here.
I know, probability doesn't really work that way and depends on the target's defenses. But given a level 11 character hitting every enemy on a 10