Calculations used in this tool are partially based on the amounts of stuff suggested by the producer itself:

• 3 gallons of water per 1 pound of fabric,
• half a bottle (or 4 ounces of powder dye) of dye per 1 pound of fabric,
• and 1 cup of salt per 1 gallon of water.

Normal people use metric system, so 11000 ml of water, 118 ml (118 g powder) of dye and 750 ml of salt for 454 g of fabric.

If you try using those suggested numbers in real life though, you would quickly see that they don't make a lot of sense. That's why I'm using different ratios (YMMV).

Notes:
1 teaspoon is 5 ml.
1 cup is 250 ml.
1 Rit Dye™ bottle is 236 ml.
1 Rit Dye™ powder pack is 1.125 oz (31.9 g).