Usually, the main concern is ingredient separation within the powder, not the water ratio. For many powdered photo chemicals, the water-to-powder relationship is effectively linear, so in principle you can mix a partial batch if you measure accurately.

However, mixed powders can separate in the package because different components may have different grain sizes or densities. That means dividing a powder kit may not preserve the intended chemical balance, especially for single-bag formulas. Gently stirring or remixing the dry powder before measuring can help, but it is not a guarantee.

Dry chemicals also do not last forever. They generally keep much longer than mixed solutions, but can still degrade from humidity, oxygen, heat, or light. Cool, dry, dark storage helps.

So: partial mixing may work, but it is less reliable than mixing the full kit as intended. Also remember there is a practical minimum volume needed to fill your tank or processing container properly. If consistency matters most, mix the whole batch; if you mix part, do it carefully with accurate measuring tools and well-remixed powder.