The game's accuracy formula is surprisingly simple.

Final Attacker Hit / ( Final Attacker Hit + Target Total Evade)

...Yes, it's that simple. So for example, 15 attacker to-hit vs 15 evade gives us 50/50 hit chances. Easy huh? But wait, it says Final, ergo it must mean something goes through it besides the baseline! And that's correct. The formula for that is

((doll baseline acc + Equipment acc¹) * time of day modifier ) * fairy buff (if applicable) * combined formation buffs² * skill buffs * fairy talents

¹ That is, from things like Red Dot scopes and Eotechs only, Night Equipment doesn't count!

² That is, the total formation buff applying to this t-doll.

Now the time of day modifier there is pretty much "1" during normal missions, so it basically never comes up. At night however, it's (1 - (0.9  * (1 - night equipment )). Effectively, a baseline of -90% acc when the unit lacks Night vision equipment (which by model number is actually an IR Laser pointer, but calling it NVGs is easier). Thus, NV equipment doesn't actually give you MORE acc, but restores accuracy you would have lost from the night battle penalty (so a 97% calibrated NVG restores 97% of the accuracy you would have lost).

Immediately however, a problem also arises. Because skills are a separate multiplier, this means things like Flare arent useful, as the -90% are counted against it still, and its a multiplier, not additive! This means that a max rank Flare skill, giving +100% more acc (effectively a multiplier of x2) thus only gives you 100% * .1 = 10%, * 2 = 20%, NOT a boost to 110%. This effectively makes it COMPLETELY USELESS.

Anyone who tells you otherwise has no idea what the formula is and can be safely ignored, as they are obviously unlearned in the ways of the game math.

Further, due to this formula, we can extrapolate that critscopes are generally MUCH better than to-hit boosting scopes unless your to-hit is completely abysmal (SMGs, and some MGs). Don't believe me? Let's go with an example with 60 tohit vs 15 evade (a common number for a lot of Sangvis units)

60 / (60+15) = 60 / 75 = 0.8

So we have an 80% hitrate. Now lets use the best possible Reddot at +30 to hit, giving us 90 to hit.

90 / (90+15) = 90 / 105 = 0.85 (the game rounds down)

So we've gone from an 80% hitrate to 85%. Basically a +6% overall increase. Compare to a critscope that makes 48% of those hits deal +50% more damage (baseline, assuming no fairies and a base critrate of 0!) and you can already see how that is MUCH better instead.

Additional Notes by Dusk#

Maths you never knew you didn't need, not until now!

The rate of dodging an attack is equal to the rate of the enemy not hitting an attack. As such, the formula is

1 - Final Attacker Hit / ( Final Attacker Hit + Target Total Evade)

Conveniently, because of maths, the formula is also

Target Total Evade / (Target Total Evade + Final Attacker Hit)

When you have both ACC buffs and Enemy EVA debuffs in the same echelon, it's can be hard to calculate the effective ACC increase or effective enemy EVA decrease, unless if you have this formula!

ACC Multiplier = 1 / EVA Multiplier

How can you get this formula?

Formula: ACC Multiplier * ACC / ((ACC Increase * ACC) + Target EVA) = ACC / (ACC + (EVA Multiplier* EVA)

x = ACC, y = EVA

a = ACC Increase

b = EVA Decrease

ax / ax + y = x / x + by

ax (x + by) = x(ax + y)

ax^2 + axby = ax ^2 + xy

axby = xy

ab = 1

a = 1/b

ACC Multiplier= 1 / EVA Multiplier

SPP-1 gives a -55% EVA debuff, which is a 0.45 EVA Multiplier, which is equivalent to a 2.22 ACC Multiplier, which is a +122% ACC buff. 

Type59 gives a -75% EVA debuff at night, which is a 0.25 EVA multiplier, which is equivalent to a 4 ACC multiplier, which is a +300% ACC buff. 

The increase in [Average Damage] caused by [ACC Increase] or [Enemy EVA Decrease] depends mostly on the attacker's ACC and the target's EVA. 

The higher the attacker's ACC is over the target's EVA, the lesser the effect of any [ACC Increase]. 

When your ACC is 4x of the targets EVA, even the strongest ACC buffs don't increase your DPS by much. Conversely, below that, every ACC buff increases your average damage by a significant, or massive amount.

However, just to remind you, when most ARs have ~60 ACC and most enemies have ~15 EVA (which puts them in the 400 section of the table), this doesn't help ARs much.

Situations when this knowledge matters are usually situations which you don't want to find yourself in, such as facing night enemies with RF/MGs (or facing future enemies).

That being said, knowing this means that you have more stratergies when team building.