@crit runes:
Are you sure those numbers are correct?
Let me math this out for you:
+33% Crit damage versus 15 AD (pure crit reds + quints versus pure AD reds + quints)
Base Case (no runes at all):
300 damage + 255% crit == 765 (it is 300 * 2.55, not 300 * 3.55)
Crit Runes:
300 damage + 288% crit == 864 (~100 damage gained per crit)
AD Runes:
315 damage + 255% crit == 803 (~38 damage per crit, +15 damage per normal)
At this (rather arbitrary) damage point, if you have a crit chance of > 25%, you'll do more damage (4 normals @+15 ~= 1 crit @+60).
Here is your more basic formula for the average base damage of an autoattack (so we are ignoring ArP, AS, etc) - and frenzy
Base AD + (Base AD * (2 + (crit damage)) * crit %)
For example, 100 base AD with a 10% chance to crit and only the crit mastery:
100 + (100 * 2.05 * 0.10) == 120.5 average
DPS gained from crit runes
ad + (ad * (2.05 + 0.33) * crit %) w/o IE -> ad (2.38 * crit % + 1) if you simplify
ad + (ad * (2.55 + 0.33) * crit %) w/ IE
DPS gained from AD runes
(ad + 15) + ( (ad+15) * (2.05) * crit %) w/o IE
(ad + 15) + ( (ad+15) * (2.55) * crit %) w/ IE
Break even point (0 <= y <= 1) - assuming no auto-crit for your first autoattack
x ( 2.38y + 1) == (x+15)(2.05y + 1)
x ( 2.88y + 1) == (x+15)(2.55y + 1)
Your item thresholds:
63 (+15 for runes) - level 1, Dorans, +3 AD mastery
76.4 (+15 for runes) - level 6, Dorans, +3 AD
135.7 (+15 for runes) - level 11, Dorans, BFS, Zeal/PD, +3 AD mastery
175 (+15 for runes) - level 16, Dorans, IE, PD, +3 AD mastery
221 (+15 for runes) - level 18, Dorans, IE, PD, LW, +3 AD mastery
236 (+15 for runes) - level 18, IE, PD, LW, BotRK, +3 AD mastery
Other stuff:
AD buff from Sona (+4 - +20)
AD buff from Baron (+40 @ 18)
AD buff from Red Pot (+15)
Before IE:
If you solve for the inequality (so that crit runes are better than AD) - again, assuming no auto-crit for your first autoattack:
1- it is always worse at AD levels lower than 138 (even with 100% crit rate)
2- After 138 AD, the amount of crit you need to break even starts dropping very quickly, then rounds off
@150 you "only" need 80% crit
@175 you only need 55% crit
@200 you only need 42% crit
@250 you only need 30% crit
After IE:
1- it is always worse at AD levels lower than 161 (even with 100% crit rate)
2- After 161 AD, the amount of crit you need to break even starts dropping very quickly, then rounds off
@175 you "only" need 77% crit
@200 you only need 54% crit
@250 you only need 34% crit
That said, everyone knows AD runes fall off, they are for last hitting. If you want scaling runes, you go ArP - and not going to do that calculation because that is freaking hard. So it isn't a surprise that crit % outscale AD.
Now what about the first auto-crit? This makes the formula somewhat more complicated:
x*(critdamage) + (number of shots)*(old formula)
Before IE:
1- It is always worse at AD levels lower than 93 (2.38x > 2.05(x+15))
2- After 93 AD, it is always better with 1 shot.
3- How about multiple shots?
General rules (since this is a complex multi-variate and difficult to see trends):
less shots -> favors crit since the upfront burst is higher and the flat AD has to "catch up" with non-crits
more AD -> favors crit, since the initial crit is larger and adding 15 to 200 is less significant than adding 15 to 100.
more crit -> favors crit (obviously), since more crit -> more crit damage
2 shots:
@100: Always worse
@150: Always better
@200: Always better
3 shots:
@100: Always worse
@150: must have 30% crit (kind of random though because 2 shots with 33% crit can still not-crit then you're behind)
@200: Always better
5 shots:
@100: Always worse
@150: must have 55% crit (less random because with 4 shots and 55% crit you'll probably crit twice)
@200: must have 18% crit
10 shots:
@100: Always worse
@150: must have 69% crit
@200: must have 31% crit
After IE:
1- It is always worse at AD levels lower than 116 (2.88x > 2.55(x+15))
2- After 116 AD, it is always better with 1 shot
3- How about multiple shots?
2 shots:
@150: must have 33% crit
@200: Always better
@250: Always better
3 shots:
@150: must have 83% crit
@200: must have 4% crit
@250: Always better
5 shots:
@150: Always worse
@200: must have 29% crit
@250: must have 9% crit
10 shots:
@150: Always worse
@200: must have 43% crit
@250: must have 23% crit
Evaluation:
Though, predictably, they scale better than AD reds, your midgame is very badly hurt if you don't rush IE first, and your level 1 is not as good as you think it is.
Also, IE + PD/SS completely neglects LS, and you have no LS in your quints. So if you do have a bad trade...