Question: how much polysorbate 20 to use in bath bombs?
Answer: 1 cup Baking Soda.
1/2 cup Citric Acid.
1/4 tsp. Finely Powdered Lavender (optional)
15 drops Lavender Essential Oil*
1/4 - 1/2 tsp. ... 
Hydrosol As Needed.
Solubilizer such as Polysorbate 20 or Solubol.