Use a spice grinder to grind the dried fish and dried shiitake mushroom into a powder.
Put the water in a large stockpot and add the konbu. Bring it to a boil. Simmer for 2 minutes, then remove and discard the konbu.
Add the fish-and-mushroom powder to the simmering water and bring it to a boil again. When it boils, remove from the heat and strain the liquid through a paper or cloth filter (a coffee filter works well for this).
If measuring the salt by weight, measure the weight of the liquid using a kitchen scale. Add 17% of this weight in salt (see here).
Alternatively, measure out exactly 2 cups of the liquid and add ⅓ cup plus 1 teaspoon of salt. You will discard some of the liquid you made and thus have less tare in the end. (Again, it’s best to use the weight measurement.)
Mix the salt in well.
Pour the liquid-and-salt mixture into a measuring cup, then transfer to a large bowl. Measure out the same amount of soy sauce and add it to the bowl. Mix well.
The tare will keep in an airtight container in the refrigerator for a few months.
