Stats Calculator
When starting to work on this page, my initial thoughts were "Hey Mystra, let's do some magic !". By the end of it, all I was thinking about was "Please Ilmater, save us !". Let's remember Cryptic's goals for the rework (they failed, hard).
Stats
Note that % Bonuses are generally rounded on display in the game, so what's on your character sheet and on many items is not fully accurate.
Your Combat Advantage Uptime depends only on your positionning. Set this to how often you surround your enemies. You can see blue stripes at your enemies' feet when you will deal combat advantage damage to them.
While you can simply add up all of your +% Damage Bonuses and Buffs together (though the total is not written on your character sheet, so have fun looking for all of them ! You can alternatively calculate it by dividing the damage you deal in the game by the damage written here with 0% bonuses, you'll find a number that's 1.something, that something is your +% Damage Bonuses), each Damage Mitigation Bonus and Buff has to be written seperately (because they stack multiplicatively and not additively : 2 damage mitigation buffs that would reduce damage by 50% would not make you immune to damage, but would reduce damage by a total 75%, which means you would only take 25% of the damage dealt to you).
Enemy Percentages
Note that Bosses have a Combat Advantage Uptime of 100%, while other monsters only have combat advantage against you when they surround you (you can see red stipes at your feet when you take combat advantage damage).
Mitigation Debuffs can be all added together (two 10% debuffs will make the monster take 20% more damage). They are generally more effective than Damage buffs, because Damage Buffs also stack additively but add to your other Damage Bonuses. So, if you already have 10% Damage Bonuses and get a 10% Damage Buff, the end result is you'll deal 1.2/1.1 = about 9.1% more damage, while you'll effectively deal 10% more damage with a Debuff.
Damage Debuffs have to all be written separately, as the calculation is exactly like Mitigation Buffs. The damage you'll take with a Damage Debuff or a Mitigation Buff of the same value will be identical, regardless of how many of each you already have. The only difference is when the calculation takes place and who is affected : Damage Debuffs are generally more effective on a single enemy hitting allies spread everywhere, while Mitigation Buffs are generally more effective against many enemies, or ones that can't be reached, and on allies in an area closeby.
Damage
Fill your Stats, or use Custom Stats.
Generally speaking, Mitigation Debuffs are better than Damage Buffs. You should always use a Debuff Artifact in group content, whenever possible (note that same artifact debuffs don't stack together, so try to use a different one from your teammates). That said, your total Damage Bonuses + Buffs stack multiplicatively with your Mitigation Debuffs, so if you have lots of Debuffs in the group/trial, a Buff (which affects everyone, otherwise it's pointless) can sometimes improve damage more than a Debuff : with 20% Damage Bonuses and 40% Mitigation Debuffs, you'll deal 1.2*1.4 = 1.68 times your regular damage, while you would deal 1.3*1.3 = 1.69 times your regular damage by replacing a 10% Debuff by a 10% Buff in that case. That can happen to newer players with low Damage Bonuses, but also in high end trials when everyone uses a different Debuff artifact.
Stat Debuffs are fairly small debuffs, but everything helps. The most important ones are Awareness (as long as you have combat advantage against your target) followed by Defense. You can modify the stats of the monsters to simulate a Stat Debuff.
Damage Mitigation
Fill your Stats, or use Custom Stats.
Effective Hit Points (eHP) are the amount of unmitigated raw damage (before crit, combat advantage, or any sort of damage mitigation) that you can take before your character becomes unconscious. You can find the raw damage of an attack in the chat logs between parenthesis when you get hit by a regular attack (non-critical, without combat advantage or damage debuffs).
Calculate Raw Damage
To calculate the Raw Damage from an attack that is Critical or with Combat Advantage (or both), input the damage taken (between parenthesis) that you can find in the chat logs. This requires you to fill your defensive stats beforehand (you can use custom stats), including eventual damage debuffs on your attacker. You can then compare this number with your eHP, to know if you can survive this attack in different circumstances : if your eHP is lower than the damage in a particular circumstance, you'll die in a single hit when that happens. For instance, a lvl 1 Damage Dealer would survive 49999 Raw Damage (with 1HP if the damage is critical, or regular with combat advantage, or critical and deflect with combat advantage), unless the damage is critical and with combat advantage, in which case they will need a friend, a scroll, or a campfire.
Healing
Fill your Stats, or use Custom Stats.
Item Comparison
You must fill your Stats above to get results relevant to your character. Fill the stats of your currently equipped item, then add items to compare the Damage, Mitigation, and Healing you'll get with them.
Note that this item comparison is not perfect, and it will never be, because it doesn't take into account Recharge Speed nor special effects (which may be a non negligible part of your build). A full simulation of your rotation, data on your powers, and your whole itemization, would be necessary to give 100% accurate results (and the code of the game because there are weird things going on). But to compare the damage of your powers with one item or the other, that's good enough.
This is the most complicated tool I've worked on so far. I've been working for the past weeks, day and night, on this one. Too many inputs, too many interractions, and that forte stat... It's been exhausting, but I hope you will enjoy fiddling with this one ! Feel free to give me your feedbacks !