Re: Zercher Squats
You say you want to find your "comfort zone" with respects to the weight? IMHO progress is made when you push slightly beyond your comfort zone in training. The more technically proficient you become, the harder you can push before hitting the point of diminishing returns.
What weight is "just right" for Z squats (or any strength movement, really)? Whatever weight makes you feel like you could not really do any more for the given number of reps and maintain good technique.