Based on the discussions with Al "CaptainGeko" Rivera, in this STOked podcast, and the PDF for 'Accuracy vs. Defense' that can be found accompanying that podcast, I would like to posit a (heavily-tested) theory on how Accuracy and Defense work in relation to each other.
For starters, let's look at what Accuracy and Defense are, and how they are modified:
Accuracy is the primary factor in determining the effectiveness of the attacks of your ship's weapons.
Accuracy is derived from several sources:
The Accurate Trait for Captains (+10% Accuracy to ALL of your ship's weapon attacks)
The [Acc] modifier on Uncommon or more rare weapons (+10% Accuracy to THIS weapon's attacks, does NOT apply to any other weapon)
Defense is the primary factor in preventing enemy attacks from outright hitting your ship.
Defense is derived from several sources:
Your ship's speed - The faster that you travel, the greater your Defense rating, up to a maximum Defense at an Impulse speed of 24.00 (NOTE: Being stopped at Impulse speed 0.00 will give your ship a -15% Defense penalty.)
The Elusive Trait for Captains (+10% Defense at all times, as long as your ship is moving)
Some items (e.g. 2 pieces of the AEGIS set, the Subspace Field Modulator, etc.) - Can be permanent, conditional increases (AEGIS) or temporary boosts (SFM).
Some powers (e.g. Attack Pattern Omega, Evasive Maneuvers, etc.)
How do Accuracy and Defense relate to each other?
First, all ships have an inherent 100% Accuracy (1.00 for the STOked spreadsheet) and an inherent 100% Defense (also 1.00 for the STOked spreadsheet).
Second, all weapons calculate their inherent Accuracy bonus independently. Thus, no matter how many [Acc] modifiers one of your weapons may have, they only apply to that specific weapon. The Accurate Trait for Captains, however, applies to all weapons (as if they each were gaining an extra [Acc] modifier). For example: if your ship has 4 Beam Arrays, 1 of them with [Acc]x2, and the other 3 with no modifiers, only the attacks by that one BA with [Acc]x2 will benefit frorm the +20% Accuracy bonus, while the other 3 will use your unmodified Accuracy rating.
Third, the baseline for 'To-Hit Chance' (i.e. how often your weapons will Hit or Miss their targets) is: 100% Chance To-Hit, if your Accuracy = the target's Defense. Therefore, if you are shooting an unmodified attack at an enemy ship that is moving as slowly as possible (to negate the Defense penalty for being stopped, but not enough to gain any bonus Defense), you will hit them 100% of the time.
Fourth, by using the STOked spreadsheet (and, again, assuming that the 'baseline' Accuracy and Defense values for all ships is 1.00, and each +X% is converted to a decimal value, e.g. +10% Accuracy would equate to .1, +50% Defense from speed would equate to .5, etc.), you can see the sliding scale of To-Hit Chance, as the difference between Accuracy and Defense increase. The minimum possible To-Hit Chance is 25%, but the realistic To-Hit Chances are typically in the 40-70% range (Escorts typically have higher Defense bonuses from speed, thus, are a little harder to hit, and Cruisers typically are slower and get hit a little easier).
Fifth, what happens when your Accuracy is greater than a target's Defense (usually only possible if you are able to stop your target, using Tractor Beams or Beam: Target Engines, etc.), well, a few things:
You still have 100% chance to-hit your target.
You gain a bonus to Critical Chance.
You gain a bonus to Critical Severity.
The bonuses to Critical Chance and Critical Severity depend on how much greater your Accuracy exceeds your target's Defense (this is referred to as 'Accuracy Overflow') and can be tracked on the STOked spreadsheet (NOTE: The decimal values in those columns work the same as for Accuracy and Defense, and convert to %'s)
Then, just for an example of comparing Accuracy vs. Defense:
Let us assume that you are firing a weapon with no inherent Accuracy modifiers, and your opponent is moving and has a +50% Bonus Defense rating from his ship's speed. On the STOked Spreadsheet, you would enter 1.00 into the Accuracy box in the top-left of the sheet, and you would enter 1.5 into the Defense box.
You will see in the "Diff:" box: -0.5 (because the target's Defense is higher than your Accuracy), and below that, you will see to boxes: "If Diff Negative" and "If Diff Positive" In this case, the difference is negative, so you would look to the right of that box, and see: "0.666666667" which is your To-Hit Chance... Meaning, you have a 66.67% chance of hitting your target with any weapon shot (i.e. 2/3 of your shots should hit the target). (NOTE: This also illustrates that you do not simply subtract your target's Defense score from your Accuracy to determine your To-Hit Chance.)
On the flip-side, assume that you still have no inherent Accuracy modifiers, but your target is stopped (and taking a -15% Defense penalty, or -.15), plugging in 1.00 into the Accuracy box, and .85 into the Defense box, you get 0.15 as your Diff score. Since this is positive, you would look in the If Diff Positive box, and get "1.13..." as your result... Meaning you have 'Accuracy Overflow'... You have 100% chance to-hit your target, and, if you look in the lower-right hand portion of the spreadsheet, you can look on the Accuracy Overflow table, and look at the entry for 0.1 (since your overflow is 0.13-ish)... You would gain a little more than 1.25% Critical Chance, and a little more than 5% Critical Severity.
Hope that helps clear up some confusion, and, if you have any questions, I will do my best to answer them here,