How is Critical calculated
Lets say you do 50 dmg regularly. You have an item that boosts 50 dmg so you have 100 total. If you were to crit, would just the default dmg multiply, or all your dmg together. For example, would you do 200 dmg (100 x 2= 200) or would you do 150 dmg ([50 x 2] + 50= 150)?
also, does critical rates round down? My runes give me a 2.88% boost in critical, but in the rune book says it is 2.8. 
The rounding down might just be the launcher client being strange. A lot of things are float values in this (abilities that say they have an exactly 8.1 second cooldown may be 8.14, etc). Critical damage is applied after everything else, so if you do 50 base damage, have an item that adds 75 damage, an effect that applies your 75 AP to your next auto attack, and critical, it'd be 50+75+75, 200, x2 (assuming no critical damage modifier).

critical damage is 200% or 100% more therefore 100 damage is a crit of 200 damage and so on and so forth
as for the runes ive noticed this as well but i believe its not that it rounds its more that it just does not show that very very minor bit after you get so far into the % bonus on crit damage 
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