Two different meanings of a dilution ratio
A 1:9 product-to-water ratio has ten total parts. For a 1 L batch, it needs 100 mL concentrate and 900 mL diluent. Product-to-total 1:10 describes the same quantities. Product-to-water 1:10 has eleven total parts, so the concentrate amount is about 90.91 mL.
In the same-basis concentration mode, stock concentration times stock volume equals target concentration times final volume. A 20% volume stock diluted to a 5% volume target needs 250 mL stock in a 1 L final batch under the additive-volume model.
