Buy chlorine strips if you want to mix up 50 ppm. If you want the math, think about it like this:
1) get the amount of pure chlorine needed. so if it is 50 ppm, that is 50/1,000,000 x 1 gallon = .00005 gallons of pure chlorine
2) figure out how much bleach you need (assuming you are using bleach). Bleach is 5.25 % in the common household variety. Meaning 1 gallon of bleach is .0525 gallons chlorine (5.25% x100=.0525)
3) Divide for amount. .00005 gallons/.0525 = .000952 gallons of bleach. Which is 3.6037 ml
But buy the strips, it's hard to get this exactly right with just bleach.
Also, I would not use bleach for this application. And dipping in boiling water is not an effective means of achieving 5 log reduction. Sorbate or citric is better. Citric also has the advantage of chelating oils in herbs, which preserves a tad more flavor.