The logic is based on the standard industry guideline:
Adding 1.0 gram of Tartaric Acid per liter of wine will lower the pH by approximately 0.1 units.
This ratio is the most common baseline for grape must and juice, accounting for the natural Buffering Capacity of the fruit.
Identify the distance between your starting point and your goal.
The Math: $Current pH - Target pH = \Delta pH$
Example: If your wine is at 3.70 and your goal is 3.40, your "Gap" is 0.30.
Because our rule says a 0.1 pH drop needs 1.0g/L, we multiply your "Gap" by 10 to find the required grams per liter.
A 0.1 pH drop = 1.0g/L
Example: For a 0.30 gap, you need 3.0g/L ($0.30 \times 10 = 3.0$).
Finally, convert your total wine volume into liters and multiply it by that dose to find the total weight.
The Conversion: 1 Gallon = 3.785 Liters.
The Final Step: $(Gap \times 10) \times Total Liters = Total Grams to Add$.
Grams to Add = $(Current pH - Target pH) \times 10 \times (Gallons \times 3.785)$
Every wine has a unique Buffer Capacity (its resistance to pH change). Grapes with high potassium or specific varietal traits may resist pH changes more than others.
Best Practice: Even with this conservative formula, it is best to add 75% of the calculated dose, stir thoroughly, and re-test after 24 hours before adding the final remainder to "fine-tune" your result.