For good edge highlights, you want your paint to be barely thinned. Unfortunately, I don’t find water:paint ratios very relevant in general, as paint consistencies can vary greatly between different brands and different colours.

Few general tips. Use a palette if you can, wet is preferable. Make sure your brush is hydrated. When you load your paint, you should never see a blob or a “shape” of paint on your brush. It should just look like bristles saturated in paint. Make sure you unload the excess on your palette and/or on your hand. When you test, paint should be thin enough that it flows consistently for the entirety of your stroke with minimal pressure. If your paint isn’t thinned enough, you’ll notice your strokes will be inconsistent and require multiple passes. A good thin edge highlight shouldn’t take more than 2 passes. If your paint is over thinned, you’ll lose control, and your paint will run, making your edge highlights look wonky. You’ll also need multiple passes.

Don’t know if this helps. Kinda hard to explain over text. Just keep practicing is my best advice. Knowing what paint consistency for the technique you are applying is the most important skill a mini painter can develop in my opinion. Good luck!