yes differentiating between .25-.50ppm cupramine can be difficult no matter which test kit.
if you dosed following the directions for .25ppm(1ml/10gal), it should be fairly accurate at .25ppm. especially if you do not have rocks/sand absorbing the copper.
i run a 29g QT. if i ever have doubts of what my copper level is at, I just do a 5-10g WC with new SW that is dosed at .5ppm. Doing so, you wont run a risk of overdosing the fish.
Either that or I add another 1ml shot of cupramine which is equivalent to +.08ppm(for my QT tank). For a tang or angel, I feel comfortable with doing that, not for a butterfly, anthias, etc.
I've put two achilles(3" and 4", at separate times) thru 3 weeks of .5ppm cupramine without issue. a PBT and a Blue Hippo too.
Great to hear your achilles did well without trouble. The test kit is reading closer to .50, but not quite there. I have been counting the doses in both drops and ml. Question is, what is the minimum effective copper dose to eradicate ich?