Introducing Newly Purchased Fish To A Tank

The 3 ppm ammonia during cycling is to make sure you've grown more than enough bacteria. It used to be 5 ppm ammonia.
But saying a fish produces 0.02 ppm per day is misleading. That just means .02 g ammonia in 1,000,000 g water (ie 1000 litres). It is not a measure of the weight of ammonia produced in a day, just the amount of ammonia in relation to the volume of water it is dissolves in.
It takes 10 times more ammonia to make 1 ppm in 1000 litres water than it does to make 1 ppm in 100 litres.