Substitute Hon Dashi for Bonito
Hondashi isn't really a substitute for the bonito flakes. It is a concentrate to make dashi...sort of like a bullion cube.  It is made of msg, salt, sugar, yeast, and dried bonito.  In general, one would use about a teaspoon to a cup of water to make dashi.  If you are beginning with prepared dashi, and looking for more umami, I might just add a bit at a time (1/4 tsp) and taste until you are satisfied.  Alternately, you don't need bonito to make dashi.  It can be as simple as kombu and water.  So, this might, in part, depend on your use.