• To Hit : H (Your to-hit as given by charbuilder)
• Level : L (Your Level)
• Bow Dice: B
• Quarry: U
• CritChance: C
• CritDamage: D
• Static Modifier: S

HitChance (T) = 1-((L+14-H-1)/20)-C

Odds of Quarry (Q) = 1-( (1-T)^2 ) * U + 1-( (1-C)^2)*max(U)

Average Damage = ( T*(B+S) + C*(max(B)+S+D) )*2 + Q

In english: Calculating twin-strike without quarry is trivial, you simply apply the average damage calculation as presented here and then calculate the odds of hitting at least once and multiply that through the quarry. I will assume that you are competent to add misc bonuses into your hit modifier and your static modifier.

In terms of optimization, for double-attackers, getting accuracy and the staticMod as high as possible is key. Hunter's quarry is "meh" in comparison.

Edit Running the numbers provided from your comment with the following:

If it's not to much to ask... what would the average damage for Twin Strike be for: Ranger, Level 26, To-hit +32, Weapon Ench +5, Weapon Focus Feat giving +3 damage, hit chance 10% (epic tier feat making crits 19-20), Quarry 3d8, and +2 to damage (due to Bracers giving +2 item bonus to melee attacks). He's using 2 bastard swords (d10 damage). –

We start by listing assumptions and requirements.

Requirement: Does a twin-bastard sword ranger do excessive damage?

Assumption: Excessive damage will be defined as being able to drop a standard monster in less than two rounds, as per the DPR king thread.

Assumption: PP and ED play a significant role in this calculation. Therefore we must calculate damage with and without an optimal PP and ED.

Assumption: An optimal PP is one that is sky blue here as is ED

Assumption: Weapon choice has a significant impact, therefore we must also perform these calculations with and without magical weapon.

Assumption: Stated +32 to hit resolves into an 18 strength at level 1 pre-racial or ED. Therefore, I will assume a +2 due to ED, and not apply racial cheese.

Assumption: Feat choice has a significant impact, therefore I will assess the damage with and without the paragon-tier prime * feats. I will not perform further calculations as to frostcheese or other damage strategies.

Therefore:

Build 1, semi-competent damage: Half-orc, two-blade ranger, Heavy weapon expertise, bastard sword prof, Stormwarden PP, Indominable Champion ED (Boring as *, but whatever), Lethal Hunter, Heavy Blade Mastery, Called Shot, Prime Punisher, Slashing Storm, Weapon Focus (Heavy Blade), Iron Armbands of Power (Heroic Tier?)

There are so many more optimizations possible, but this is a good "Not too glassy" option leaving many feat, items, and so-forth.

Character builder produces the following attack: +32 v. AC // 2d19+10 damage,+3d8 quarry, +5 to damage rolls due to called shot, prime punisher, +1 to accuracy due to prime shot.

The complete expression to be evaluated is, therefore:

(32 accuracy, AC of 26+14, extra -1 due to prime shot)

Base damage, before quarry. (1-(26+14-32-1-1)/20-.1)*(5.5*2+10+5)*2+.1*(10*2+10+5+3.5*5)*2 = 41.700 Rounds to drop 1 enemy, before quarry = 5.5 = absolutely pathetic for a striker. Minimum recommended striker rounds = 4

Quarry damage, calculated correct follows the following pattern: Odds of hitting the first time * quarry + odds of critting the first time * quarry + (odds of hitting the second time * odds of missing first time) * quarry + (odds of critting the second time - odds of missing the first time) * quarry.

Odds of hitting: (1-(26+14-32-1-1)/20-.1) 60% Odds of critting: 10% Odds of missing: (26+14-32-1-1)/20)=30% Total odds =1

.6*4.5*3+.1*8*3+.6*.3*4.5*3+.1*.3*8*3=13.65 expected quarry damage from one twin strike

Total DPR:

13.65+41.700=55.130 (8*26+24)/55.130=4.208

Not quite in minimum striker level damage.

Extra DPR 2*Dex (stormwarden, assuming both 11 and 16 come into play) + Wis (slashing storm)= +8+8+2 = +18

55.130+18=73.130 (8*26+24)/73.130=3.172

Just about right for a semi-optimized ranger at-will.

Dropping prime punisher and called shot gets twin-strike DPR to: 32.6, 13.257 quarry damage

32.6+13.257+18 = 63.857, 3.633 rounds, just right.

Dropping stormwarden:

32.6+13.257+2 = 47.857, 4.847 rounds, unacceptable.

Therefore, based on the parameters you described, the ranger's damage probably fits into the low end of the "striker damage" window or below and should be allowed into your game.

This answer is a conservative justification of the low values presented in DPR King 2.0:

Squares       Friendly     Unfriendly
4  (2x2)       1.2         1.1
9  (3x3)       1.45        1.3
16 (4x4)       1.8         1.5
25 (5x5)       2.25        1.7
36 (6x6)       2.8         1.9
49 (7x7)       3.45        2.1
64 (8x8)       4.2         2.3
81 (9x9)       5           3
a 9x9 is the cap.

After some deliberation, I agree with these assessments. Given that enemies will be avoiding bunching up save for the opportunity to flank, the heavy discount on unfriendly AoEs seems to correspond to how often there will be a combat where:

1. There are enough enemies

Given that the number of enemies in a fight optimally decreases with respect to time (you don't want to leave everyone on 1 hp until the last round) blasts and bursts have a decreased time where they can be effective.

Given that a non-trivial fraction of the fights are versus solo, elite, or small numbers of people, fulfilling the parent condition is difficult.

2. They are close enough without a party member in the way

Enemies will maneuver to avoid AoEs, just like party members. The most likely people to catch are the meleers flanking in the dogpile/scrum/charlie foxtrot, whatever you want to call the ball where all the melee happens. Targeting party unfriendly powers is hard, and almost certainly cannot happen every round.

Now, with that said, there are all kinds of ways to virtually expand the size of blasts and bursts, and anything that just adds "one square" onto a blast or burst can be said to upgrade its size for purposes of this chart.