It's all by feel.
The most common mistake guys make is trying to "twist the pushrod" while trying to get to zero lash.
The best approach is to back the adjuster way out, then move the pushrod axially up and down while sneaking up on the adjuster until you don't feel any free-play anymore. You will feel when the pushrod touches the lifter plunger. Once you get to that point, you dial in your preload and lock it down.
A lifter can be pumped up or bled down and the pushrod seat in the lifter will be in the same spot. There is a spring under the plunger and it makes sure that the pushrod seat is always in the same spot, whether or not the lifter is "pumped up" and full of
oil.
Preload is usually around .030-.050" of plunger depression. That's generally around a 1/4 to 1/2 turn of preload.