Are you saying that you normally make ganache with the chocolate weighing double the weight of the heavy cream? Unless you are using milk chocolate, that would be a ganache that is nearly impossible to spread on the cake and would be very firm for eating.
Classic ganache proportions are equal weights heavy cream and dark chocolate, in many instances (depending on which chocolate), this will make a ganache that is quite thick and can sometimes be a little difficult to spread on the cake, which is why Rose puts a little more liquid than that.
I was just recommending that if you reduce the heavy cream that you not go lower than equal weights cream and chocolate.
All the above is for spreadable frosting. Are you trying to make a glaze that will set to a firm or hard texture? Then you might go something like 2:1 chocolate : cream and use while still pourable.