I cheat; I consider:

1/8 teaspoon per 5 gal (US) to be 20 PPM.
1/4 teaspoon per 5 gal (US) to be 40 PPM.

Technically that is 10% low, but I figure there are enough variables
that affect this such as age of the actual chemicals that measuring
with a balance is just not worth the effort.

I use the spoons correctly; I over-fill and wipe them flat with the
side of another spoon. It works for me. I'm not saying you shouldn't
do the calculations, just that this is one of the areas where close has
been good enough for me over the years.

Joe