You can estimate how much is in your water using this calculator. You can put in as many water changes as you like and play with how much of what you want to add then see what the build up of nutrients might be depending on uptake.
One reason dosing is done on alternate days is because iron can be bound by phosphate. Some find that acidic water doesn't support that reaction and some report that plants do just fine dosing the two a little apart instead of a whole day apart. Another reason is so absent minded people remember to dose and daily is easier to remember than every other day.