I'd just use 30'. When we were in Haines we drove to the terminal and talked to them. At that time there was no need to pre book but we did arrive a little early to be near the front.
We were concerned about the ramp situation etc since there was no information on the website and the tides can be extreme. While they are well aware of rig clearances I'd advise anyone to proceed with caution and if concerned stop and they will quickly be there.
Our rig is low and they checked the clearance on and off.
I was amused when later in 2013 the Skagway floating dock sank (it's between shore and the ferry) due to a fresh water supply problem. I'm sure many weren't so amused since it took a number of weeks to refloat. But that is all part of the Alaska trip experience.