I also mix black and green teas often. As far as the 50% rule goes, I've been using plain green tea for more than 20 years without problem. Don't use Earl Grey however, or any other flavored tea. It contains oil of bergamot, and essential oils of any kind can interfere with the metabolism of the pellicle. Herbal teas should be avoided too, with the exception of yerba mate and, I've read, rooibos.