I've made kimchi a couple of times. As others suggest, the main issue with a lot of American recipes is that they use volumetric units (cups, spoons, and what not) to weigh out ingredients. The main difference between kosher salt and normal salt is density. And probably no two salts have exactly the same density. Meaning that these recipes are super imprecise.

So, the good news is that precision doesn't really matter for a lot of these recipes. But if you want to be precise, use proper weight units and get some proper tools to weigh things out. 2.5% is a good target (by weight) for a brine. I use precise weights when making bread. When making kimchi I don't weigh out anything.

Here's what I tend to do too make kimch:

1) get cabbage and use lots of salt to draw out the moisture. More is better. The goal is to draw out water. You cannot use too much salt at this stage. Use cups, buckets, whatever makes you happy. It does not matter.

2) after 40 minutes or so, wash the cabbage; most of the salt will go down the drain at this point. This is why the amount of salt doesn't matter. The remaining cabbage will taste slightly salty but not overly. That's because some of the salt will have made it into the cabbage through osmosis. Nature will get the ratio right for you.

3) prepare a paste with sugar, some salt or fish sauce (season to taste basically), some starch to thicken, mix in the chili flakes and your other ingredients. Use Korean chili flakes if you can get them but you can fall back to other chili flakes. You want mild heat, don't go too crazy here.

4) mix everything together and put it in a jar. There should be enough juice coming out of the cabbage to cover the ingredients. But if not, add some water. Obviously use water without chlorine. Or put differently; use water that tastes nice when you drink it. Don't put stuff that doesn't taste nice in your kimchi.

5) Let it ferment. You should see fermentation happen within a day. Activity will taper off after a few days. There is no right or wrong here. Some like it fresh, some like it funky. Ferment longer for more funkiness. I usually go for about a week.

I loosely follow Maanchi's recipes. But I've watched a few other ones on Kimchi. Brad Leone is highly entertaining and he ferments all sorts of stuff. The main thing you can learn from him is that fermentation is easy and that there really is no right or wrong here.

It's a natural process and all you are doing is letting nature do it's thing. It's pretty hard to mess it up actually. The main thing to understand is the optimal salt to water ratio in a brine (and a lot of other food) is about 2-3% by weight. But you can be off by 50% and it won't really matter much. Anything too high and you kill the yeast and the bacteria and they'll be less active. Anything too low and they'll be less active. You can just go by taste here. If it tastes too salty, it probably is.

Kosher salt is a US thing. But of course kosher food is a Jewish thing and Jews of course emigrated pretty much everywhere. Though probably not a lot of them ended up in Korea. All it means for salt to be kosher is to have no additives. So, get some salt without additives and weigh it out and you'll be fine. I've used with and without and honestly it doesn't make a huge difference. Use what you have.

For authenticity, use Korean ingredients and get all the good stuff. But bear in mind that this is food that was made for centuries by people without any modern tools. It's not that hard.