Yes - bearing adjustment is very much trial-and-error.
The bearing balls must be located so they are restrained from moving in any direction except around the race.
Do do this, the mechanic must "preload" the bearing so there is slight compression all the time, but not so much that the bearings cannot spin freely.
This process is hard to describe but can be felt directly.
TOO LOOSE: detectable play in the bearing in the left/right or up/down direction.
TOO TIGHT: bearing is hard to turn and resists correct operation.
Make it easier on yourself by:
- Disassemble and clean the cups, cones, and all balls before assembly.
- Inspect after cleaning - if there's pitting on the cone or cup consider your options. If the balls are not shiny and round then simply replace them. Bearing balls are cheap.
- Use grease to hold the balls in place while reassembling
- Use the right spanners - cone spanners exist and are especially thin to make this task accessible. If you're using normal spanners, or pliers then it becomes harder.
Ideally you'd tighten the cone nut till it's perfect, then put the locknut on . But the locknut always interferes with the placement of the cone. At this point its a matter of adjusting things by an eighth of a turn.
If you can't find the sweet spot between tight and loose, its okay to have a very small amount of play more than having resistance that fights you and chews up the bearing races over time.