Work out the factor
Divide your desired ingredient amount by the corresponding amount in the original recipe. If the original uses 2 litres water and you need 3 litres, the factor is 3 ÷ 2 = 1.5.
If that original recipe has 80 g salt, multiply 80 × 1.5 to get 120 g salt. Water and salt both increase by 50%, preserving the water-based 4% ratio.
Include food for a food-based ratio
For an original batch with 500 g food, 1,000 g water and 45 g salt, a factor of 0.5 gives 250 g food, 500 g water and 22.5 g salt. The food-plus-water percentage remains 3%.
If you change the food amount without scaling the water, that is a new batch composition. Calculate it from the new food and water masses instead of assuming the original scale factor still applies.
Check before processing
Ingredient scaling does not validate a new preservation process. Your recipe may restrict batch size or specify container dimensions, cooling, treatment and storage steps. Follow those instructions rather than multiplying the process duration by the same factor.
For arithmetic, use the recipe scaling calculator. It accepts decimal factors and shows the retained percentage.