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Chlorine powder is the common name for calcium hypochlorite, also known as bleaching powder or chloride of lime. Unfortunately it quality varies - commonly between 30-35% chlorine. Because it contains excess lime you want to make it up into a concentrated 'day tank' solution before dosing, this allows the lime to settle so you can draw the chlorine solution from the top.

In terms of formulae, try this from WHO (https://www.who.int/water_sanitation_health/hygiene/emergencies/fs2_19.pdf):

W = 1000VC/S where: W = weight of powder required (grams) V = volume of tank (litres) C = concentration of solution required (% w/w) S = strength of powder (%chlorine w/w)

Commercially available chlorine solution is sodium hypochlorite. While chlorine powder decays with time, sodium hypochlorite's decay is much more rapid, so I definitely wouldn't trust what it says on the container. I would dose and measure, preferably with a closed control loop. Sodium Hypochlorite in concentrated form (10-15% w/w) is too concentrated to easily measure, hence after some dilution tests, I would recommend measuring in the treated water.

Here is the comparable WHO fact sheet for Sodium Hypochlorite: https://www.who.int/water_sanitation_health/hygiene/emergencies/fs2_20.pdf

I imagine the above is what you mean by formula. But if you wanted the stoichiometry, the dissociation formula is the same for gas, powder, granules and solution; once chlorine has dissolved in water: https://www.aqueum.com/encyclopedia/chlorination/