1. Do not squeeze the katsuobushi too hard as it will produce a slight bitterness.
2. You cannot keep dashi very long even in the fridge as it loses flavour. 2-3 days is the max, I would think. Freezing will also lose the falvour but if you don’t mind losing it, you could freeze the stock.
3. If you would like to make dashi from store bought dashi pack or seasoning, just follow the instructions on the packet which should tell you how much you need for certain amount of water.