How am I suppose to figure out when it's better to build Lethality vs Damage/Crit?

Aside from scouring the wiki and doing a lot of complicated math to figure out all the exact specific circumstances when a specific hero should build arpen vs damage, are there any basic guidelines you can follow to help you judge when one is better than the other on AD assassins or assassin-like fighters/ADCs/bruisers? If not, the devs should probably consider ways of making this easier for players to judge in-game.

Runes alone would allow you to penetrate the average base armor at level 1, which appears to be around 24 Partial runes and cunning mastery would allow you to accomplish the same thing at level 1.

You will, however, require max runes, masteries, and all three armor pen items in order to negate the average base armor value at level 18, which appears to be around 83. At that point, a % armor pen item will only help if they have bought at least one armor item or they have spells/abilities that you anticipate will be giving them armor when you face them.

Even assuming we are dealing with a relatively low armor ADC or support, who doesn't build any armor items and has no armor buffs, how are we suppose to know whether or not it's better to invest fully in armor pen vs damage and crit?

How do you know when they've built enough armor that a Dominik's Revenge will be a better overall damage than to buy a higher flat damage/crit item like infinity edge?

I think Talon is the main champion that I have this question concerning. Although I do also have concerns about figuring out this issue with some of my other favorite AD champions that I play more like assassins (Quinn, Vayne, Jax, and to a lesser extent Graves, VI, and possibly the new Warwick), but those champions start to get even more complicated because attack speed or defensive options are other variables that are important to factor in for optimal damage output. Talon is probably easier to figure out because he's so bursty that he isn't really designed to slug it out in auto attacking anyway. So it's really a question of lethality vs damage and crit.

Any help figuring this out would be appreciated.