Is a strand test really necessary, and how do I do it properly with only 40 ml of product?

A strand test is the single best way to avoid surprises, particularly because this shade develops much more strongly on grey, bleached, or highly porous hair. The 40 ml tube contains enough product for roughly 8–12 full applications (depending on hair length and thickness), so using a pea-sized amount for a test costs you virtually nothing. To test: separate a small strand from underneath the top layer (nape or behind the ear), apply the shampoo to that strand only, leave on for the minimum recommended time (usually 3 minutes), rinse thoroughly, and dry. Check the result in natural daylight. If the tone is too intense, reduce leave-on time for the full application; if it's barely visible, you can safely extend by 1–2 minutes. Also note how evenly the colour takes — patchy uptake signals uneven porosity, which means the full-head result may vary along the lengths. In that case, consider a pre-treatment with a porosity equaliser or a light protein filler before the next tinted wash.