CDR to Round Robin skills.

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

Forgive me for my newbness, I am interested in the whole round robin concept.

Is this correct: 4 chars with different Cooldown reductions can chain a skill (i.e. frost nova in sequence for longer freeze duration) ??

more or less; yes.

Or more exactly, if 4 characters all have an individual CDR of 27.27% [on the skill sheet; since total CDR is not exactly additive] then yes, in theory.

In practice, meeting the minimum isn't always enough to prevent staggering; it's better to have a bit of a buffer sometimes.

For the CDR calculation alone, this is the best site I've found so far:

http://bannedofgamers.com/index.php?cdr/

It can count passives, specific items, paragon, etc. for all classes.

Additionally I think this is one of the strongest aspects of multiboxing especially in Diablo 3. By controlling the entire party you are able to pull off very fluid combos or chains where a coordinated group couldn't get close to this efficiency.