CDR to Round Robin skills.

[Ishar]

Method:
In order to round robin a skill your effective cooldown must at least be equal to the duration of the ability times the number of characters you have to round robin with. Ergo, for 4 characters and an ability with duration 10, effective cooldown must be 40 or less.

Given the following formula for calculating the effective cooldown (cooldown - reduction) of an ability:
Effective CD = CD - CD * (CDR/100)

This yields the formula:

CD - CD * (CDR/100) = duration * Characters

apply some simple algebra:
Characters Needed = (CD-CD* (CDR/100))/duration
CDR Needed = -100 * ((duration*Characters-CD)/CD)

Feel free to correct if I've mucked up the algebra somewhere. But I'm pretty sure it's right. Additionally, as there is an ability listed with a 7.5 second CD, I assume partial seconds are preserved when Diablo 3 calculates your effective CD.

See below for table form. Spreadsheet is here. Note that spreadsheet has a slot for inputting your current minimum CDR; which yields the number of characters you require to round-robin a given ability at that minimum CDR. It is not editable to avoid defacement; just download it and use it locally.

Note that total CDR is not additive; a calculator for it is here.

Edit: Added link to CDR calculator, thanks Dev.
Edit: Updated for 2.0.5

Skill	Rune	Class	Dur	CD	1	2	3	4
Law of Valor		Crusader	10	30	66.67	33.33	0.00	0.00
Law of Justice		Crusader	10	30	66.67	33.33	0.00	0.00
Law of Hope		Crusader	8	30	73.33	46.67	20.00	0.00
Judgement		Crusader	6	20	70.00	40.00	10.00	0.00
Shield Glare		Crusader	4	12	66.67	33.33	0.00	0.00
Consecration		Crusader	10	30	66.67	33.33	0.00	0.00
Condemn	Vacuum	Crusader	3	15	80.00	60.00	40.00	20.00
Breath of Heaven	Zephyr	Monk	3	15	80.00	60.00	40.00	20.00
Breath of Heaven	Infused with Light	Monk	5	15	66.67	33.33	0.00	0.00
Breath of Heaven	Blazing Wrath	Monk	10	15	33.33	0.00	0.00	0.00
Blinding Flash		Monk	3	15	80.00	60.00	40.00	20.00
Blinding Flash	Self Reflection	Monk	6	15	60.00	20.00	0.00	0.00
Blinding Flash	Replenish	Monk	5	15	66.67	33.33	0.00	0.00
Epiphany		Monk	15	60	75.00	50.00	25.00	0.00
Serenity		Monk	3	20	85.00	70.00	55.00	40.00
Serenity	Ascension	Monk	4	20	80.00	60.00	40.00	20.00
Inner Sanctuary		Monk	6	20	70.00	40.00	10.00	0.00
Inner Sanctuary	Sanctified Ground	Monk	8	20	60.00	20.00	0.00	0.00
Frost Nova		Wizard	2	11	81.82	63.64	45.45	27.27
Frost Nova	Cold Snap	Wizard	3	7.5	60.00	20.00	0.00	0.00
Black Hole		Wizard	2	12	83.33	66.67	50.00	33.33
Ground Stop		Barbarian	4	12	66.67	33.33	0.00	0.00
Ignore Pain	Mob Rule	Barbarian	5	30	83.33	66.67	50.00	33.33
Avalanche	Snow Capped Mountain	Barbarian	3	30	90.00	80.00	70.00	60.00
Hex		Witch Doctor	12	15	20.00	0.00	0.00	0.00
Mass Confusion	Paranoia	Witch Doctor	12	60	80.00	60.00	40.00	20.00
Horrify		Witch Doctor	3	12	75.00	50.00	25.00	0.00
Horrify	Phobia	Witch Doctor	6	12	50.00	0.00	0.00	0.00
Big Bad Voodoo		Witch Doctor	20	120	83.33	66.67	50.00	33.33