The common ratio between dashi and miso is 15 grams of miso per 1 cup of dashi . Over boiling causes flavor and aroma loss, so don't over boil after mixing in miso in to soup.
Timing is essencial depending on the ingredient.
When using root vegetables cook them in dashi first, when soft stop heat add miso and heat again and stop just before it comes to a boil. On the other hand, when using tofu , wakame or green onions add after mixing miso . Miso has many varieties of sweet, salty, white and red. Select the appropriate miso to match ingredients. Also several kinds of miso mixed together enhance soups flavor.