Is there a directive with regard to the use of a tell-tale dye in the preparation of capsules, e.g. Percentage of active substance on total mixture < = 1%?

An answer to your question is reflected in the lines of the monograph of the Eur. Pharm. On the control of uniformity of distributed preparations. This test was introduced because uniformity of mass is not a guarantee of uniformity of content (due to poor mixing). Recently the criteria were changed and a much larger number of capsules will have to be subjected to the test of uniformity of content. The old criteria were that a tablet or capsule with a content of active ingredient as a dye we use for:

1. a trituration 1 on 500 or 1 on 1000 riboflavin 0.1%
2. a trituration 1 on 50 or 1 on 100 red iron oxide 0.1%