As a casual KE flyer (2-3 times a year to the US and around Asia), I feel like the rule of thumb should be "expect a full flight." Every time I have called early to book (at the opening day window for my flight), I have always gotten the seat I wanted. Just know that KE keeps the front half of all planes for their Skypass Morning Calm Members and Skyteam Elite/Elite+, so you will end up in the back half otherwise. Exit row? EVERYONE has to get that at check in.

Good luck!