For me:

You need to see your target to gain the advantage of being unseen by them when you attack. Basically, if two people are fighting in a fog cloud, they both have disadvantage to attack. Otherwise you get situations like being more accurate shooting at the long range of your bow if you first walk into a fog cloud, blinding yourself and becoming unseen.

I've also added a baseline range at which you can automatically detect invisible creatures that aren't hiding (just their location, you still have advantage/disadvantage if you can't see them), which is passive perception / 2 * 5 feet around you. I also use this range as the distance you automatically detect vocal components to spells being cast, if the environment isn't covering it up. It's not really a strict rule since circumstances will change things, but it's nice to have a base value (numbers may still need tweaking).