I live in a very humid tropical climate in sunny Nth Queensland Australia
I use the 2:1 ratio and it's perfect here. I've also done a near 3:1 and I loved the results too. If I go any lower however, like 300mL cream:300gchoc it's soupy and doesn't like me at all!!
I heat my cream up till near boiling ( I see steam maybe a small bubble or two then take it off the heat). I have done it before where I've left the cream on the element and walked away ( at work turned away to tend to a customer) come back and it had boiled right up to the top of the pot!!!!!! It still turned out. I take the pot of cream off the element and add ALL the chocolate in ( double to that of that to cream) and stir and stir and stir till it's thick glossy and smoooooth.
Also another great tip is too keep your chocolate at room temp all the time. If your adding cold choc to boiling cream your lowering the creams temp down too fast I reckon and may need to put the pot back on the element to keep it warm...which to me is scary and can lead to burning if your not babying it.
Hope my tips help