Do you have a preferred balance point with a normal 58" cue? If so, calculate the balance point for a 66.5" cue using the same ratio.
For example, assume you like an 18.75" balance point for a 58" cue. 18.75 is rather neutral. The ratio from rear of the cue is 18.75/58, which is .32328. Multiple that by a longer cue, 66.5, and you get a new balance point of 21.5".
Depending on where most of the extra length is in relation to your stance/grip/bridge, that starting point would be tweaked of course. Since most of the time extra length tends to be in the front, 21.5" would be very forward balance. My suggestion would be start with 20.75".
Not meant to contradict Martin...just a mathematical way to get a ball park initial number.