Here is Hervey Lovell's formula from my 1938 copy of "Elemental Gunsmithing":
Bichloride of mercury- 1 oz.
Potassium Chlorate- 1 oz.
Potassium Nitrate- 1oz.
Spirits nitre- 1.5 oz.
Distilled water- 1 pint
"for less than $1" in 1938.
Researching the modern name of Spirits nitre is up to you.
Since the product you have is pretty certain to have potassium nitrate in it already, adding more should not cause a disaster, JMHO.
Buying any chemicals today will likely be an exercise in frustration; all the "good stuff" has been removed from kid's chemistry sets!
