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....