Why would you extrapolate based on a log curve? Since we know for a fact there's a difference between regular and epic scale, why not fit two linear curves--in which case the ISD would be 84.5 inches long?
In my opinion, a log curve is the proper way to scale ships.
If they were to come out with a ship that is 70m long in canon, which scale would you use, the one for huge ships, or the one for small ships? Probably one in between - which is exactly what a log curve gives you.
Edit: If you calculate a log-curve from the 2 huge ships we have so far, ignoring the small ships, you get the following extrapolated measurements:
- Vigil-class Corvette: 14.1"
- Victory I-class Star Destroyer: 21.5"
- Imperial Star Destroyer: 24.9"
- Death Star: 50.3"
- Alderaan: 77.5"
Edit 2: Also, cool tidbit, human perception is logarithmic .
Edited by Klutz