It doesn't matter what size container you're working with or how much material you're working with, as long as each part is equivalent to the others. So to take your example, say we're mixing a product at a one part cleaner to three part water dilution ratio, and you have a gallon bucket to measure all the components. That means you're filling up the bucket three times with water, and one time with the product, resulting in a solution at the proper ratio. Always add water first, and then the cleaner.
A part can be a 1-gallon container, a 5-gallon bucket, a pint, or even a capful. If you just need a tiny bit of your solution for the example above of one part product to three parts water, use three capfuls of water and one capful of product.
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