You can think of crit as a multiplicative boost and to decide if it's better to invest in crit, or the base damage being boosted by crit, or a crit bonus.
I like to always dumb things down into easy-to-understand numbers.
100 base damage. 5% chance to crit. Default crit bonus is 100% (double damage).
So you have a 5% chance to do double damage.
You hit 20 times for a (base) total of 2000 damage (100 per hit, 20 times, no bonuses included).
In practice, ~1 in every 20 attacks will do double damage, so you've done the same amount as you would have done in 21 attacks, because 1 of them did double. In this scenario, it is equivalent to getting an affix on your gear or gems that says "do 5% more damage".
So if we tweak it up a bit - what if it's a 10% crit chance, everything else the same?
Now you've hit 20 times, and 2 of them were double damage so a total of 2200 - which is 10% more damage than 2000. When we only change crit chance and we have no bonuses to crit bonus, the crit chance we have can be thought of as a "more" multiplier in sustained fighting scenarios.
Now lets consider a chance to crit bonus. Revisit our initial 2000 damage and base crit chance of 5%. We get 50% increased critical strike bonus, that means when we crit, it does more than double-damage, now it does 250.
So in that scenario where we hit 20 times with 5% chance to crit, we do a total of 2150 damage. This is equivalent to 7.5% more damage, which makes sense --> at 5% crit, no bonus it's 5% more damage, and we increased it 50% to 7.5% more.
Lets consider 10% base crit now, remember it was equivalent to 10% more damage. A 50% increase to 10% more is 15% more. If we revisit the 20 hit scenario, we do a total of 2300 damage, which is 15% more than the base amount of 2000.
So lets imagine you are sitting with 10% chance to crit, and 50% increased crit bonus --> is it better to increase your base damage by 10%, crit chance by 10%, or crit bonus by 10% for the "best" increase to your sustained DPS?
Scenario 1 - base damage increased by 10% (100-->110). Total sustained damage output over 20 hits is 2530 damage.
Scenario 2 - increase critical strike chance by 10% (10%-->11%). Total sustained damage output over 20 hits is 2330.
Scenario 3 - increase critical strike bonus by 10% (150%-->155%). Total sustained damage output over 20 hits is 2310.
So if you're able to get 20 hits out, you're going to see the best result by increasing your base damage by 10%.
In practice in the game, the scenario we've postured isn't necessarily going to represent how your attacks get to play out - people rarely get to sit there and put out 20 attacks in a sustained fashion, unless they have extremely high attack speed (I'm talking 10+ attacks per second).
If you are attacking or casting at a casual speed, you need to consider how many attacks or casts you're willing or able to put out in order to roll a crit and receive that crit bonus - because you can technically get 4 billion crit bonus and see a multi-million DPS number, but if you only realize that bonus once every 20 attacks, and it takes you half a second to attack, you're gonna be doing your base damage on average for 9.5 seconds until you get that bonus-affected attack out, and it will feel terrible to play even though on paper it looks huge.
It means there isn't really a straightforward answer to your question unfortunately, it is heavily reliant on playstyle, other support gems, skill choices, and your base crit, and cast/attack speed.
The way people typically do it is that you focus on normal increases/more multipliers for your damage unless you can get your crit chance to a threshold where within your usual attack timing window (ie. how long you're willing to spend to stand there and attack/cast before needing to dodge or progress) you consistently hit a critical strike. Don't focus on allocating crit bonus until then, otherwise the skill is going to feel clunky and unreliable because you won't be seeing your crit bonus benefits on a consistent basis.
For #1 & 2 - increases are calculated after conversions/gains.
So if you have 10% increased physical damage and gain 10% of damage as extra physical, and you are doing 100 physical, 100 fire, 100 cold, and 100 lightning damage:
- first you get the gain as extra, so you're doing 400 damage total, and you're gonna get 40 extra as phys. Your new totals are 140 phys, 100 fire, 100 cold, and 100 lightning.
- you do the increase of 10% increased phys, so your new totals are 154 phys, 100 fire, 100 cold, and 100 lightning for a grand total of 454 damage.
- if you slapped an embitter gem into your attack, that 40 extra phys gain would be now gained as cold instead, and the increase would not apply, so your totals would be 110 phys, 100 fire, 140 cold, 100 lightning, for a grand total of 450 damage - not too far off, but composed differently.
