I usually move the females to a CB-110 when they hit 2500 grams.
I'm not saying females that size shouldn't be kept in CB-70s but I just prefer to provide the extra room. Balls are primarily sedentary snakes, but if I they want to stretch out and move a bit I want that to be available to them.
A lot of people can't use the 110s though, a 39 inch long tub does take up some space.
Another reason I like the 110s is they are more versatile. I have one rack of them that has some big female balls in it, but it also houses some adult womas comfortably. I've kept decent sized dumeril's boas and hog islands in those tubs as well.
As for the heat tape, I know the freedom breeder racks work well with the 3" tape, but I just prefer to have it set up so that an adult female ball, when coiled, can get her entire body over the heat source evenly if she so desires. Using the 39" long tubs also allows for the larger heated area without affecting the remainder of the cage space.
I think a 3" strip, set about 4 inches from the end of the CB-110 and spanning the width would work fine too.