It appears the most widely used ratio for dark chocolate ganache is the 2:1 ratio of chocolate to cream (I've seen this ratio also recommended for semi-sweet chocolate). But we all know that not all dark chocolates are created equal. For those who have used or do use chocolate with cocoa mass that's higher than average, 60% and above, do you use the same ratio or increase the cream slightly to 'compensate' for the extra cocoa? 

I just tasted a ganache I made using the 2:1 and it's a tad 'much': slightly bitter, super thick, need to reheat it to keep it at the right consistency for spreading. Which is not a bad thing, I just feel like I might get a better result with a 1.5 : 1 ratio... but before I experiment, does anybody here use a ratio less than the recommended 2:1 for ganache UNDER fondant? 

I have 60% cocoa mass, I can't imagine a ganache with 72% or more at 2:1.

I use a 72% and I use 2:1.  I think its' fine!  In fact I have a harder time using my white chocolate even though it's 3:1.

Thanks FromScratchSF. Do you find the resulting ganache slightly bitter? I'm thinking of adding some sugar to it. Maybe that'll take the edge off. 

Yes it's bitter, but in a good way.  The brand of chocolate makes all the difference.  Sweetening it I don't think will change the underlying flavor of the chocolate, but if you want to take the edge off a bit add some vanilla extract.  

And want something to DIE for?  Instead of using cream use Guinniess or a good dark extra stout.  It's stupid good.

I use 70% chocolate in ganache under fondant, and use 1:1 ratio. It still firms up REALLY hard. The amount of cream lessens the bitterness slightly. And not to mention, at least where I live, a kilo of cream is cheaper than a kilo of chocolate, so for me it's the cheapest of all.
 

AThanks Dayti.

HAVEN'T converted to ratios but a great recipe / guide to ganache is a series on YouTube called "Inspiried by Michele's" Ganache. She has a 3 step video series for making ganache & torting /filling cakes with ganache. Her recipes break down to the following:

WHITE CHOCOLATE:  1.25 cups heavy cream (35% fat content) : 2# white chocolate

DARK CHOCOLATE:  1.25 cups heavy cream     "      "      "       : 1.33# dark chocolate

Heat chocolate to near boil (in bowl big enough to hold all the chocolate you will be using + some room to mix), add chopped chocolate, then let stand until chocolate has absorbed most of the heat from the cream, about 2 minutes, and gently swirl to blend (to avoid getting bubbles in ganache). Let stand 2 minutes more, then stir to smooth out any remaining solids. If there are still 'lumps' microwave 1 minute, let stand a minute or two, then stir, continuing in 30 second microwave bursts if necessary. Let stand at room temp over night if possible to cool completely. You should have a product that is easily spreadable, similar to buttercream consistency. If desired, you can whip the ganache by hand with a whisk or using the whisk on your mixer, beat well to get a gorgeous spreadble product. The dark chocolate will become lighter in color. This is fabulous stuff for filling or covering cakes, and takes an edge really well.  It's also a great base for fondant, and when bittersweet chocolate is used with fondant, the two complement each other beautifully. The very sweet fondant is balanced perfectly by the bitter chocolate. 

As far as adding sugar, you would have to avoid the crystals even from powered sugar.  Maybe dissolving sugar in cream before mixing with chocolate. But there are suitable semi-sweet chocolate products, and I have mixed white & dark chocolate ganache to get a less bitter end product.  It's, too, is fabulous.

"Inspired By Michele Ganache" YouTube videos are wonderful, and give you visuals no recipe can describe. They've changed my cake making processes for ever....