I had to mess with the scaling and through trial and error got to the right size by comparing to the pump head. Can't remember offhand what the value was, but somewhere in the low to mid 60s% range I think it was.
I normally throw 1 frozen 2 liter in there right after pitching and sealing it up. Pitch around 68 degress, so even if it chills to the max it would still be greater than 62 with just the 1 bottle in there. I normally use WLP001 yeast and this works out well.