It may be worthy of note that Kris Dhillon, who was my guiding light in the early days of my BIR journey and to whom I always return when I feel that I have strayed too far from the path, makes no use of either "mix(ed) powder" or curry powder (her basic spices are ground chillies, ground jeera and ground fenugreek, together with the paprika and turmeric that she has used in the preparation of the base), but she does make use of her own version of garam masala which she introduces part-way through the cooking at the same time as she adds the ground jeera and ground fenugreek. Her recipe for garam masala includes coriander, cumin, green and black cardamom, cloves, black pepper, cinnamon, bay and nutmeg, all initially whole and then ground (unroasted) until smooth before storing in an airtight container.
Maybe the OP is making an error in the reading of the terms "mix powder" and "mixed powder". In reference to BIR style cooking, when I read the term "Mix Powder" I associate it with an individual chef's own (secret) mix of single spice powders. As opposed to a proprietary "Curry Powder" mix.
All of the mix(ed) powders for which I have seen a recipe, here or elsewhere, use curry powder as one of the ingredients. Bruce Edwards, for example, uses coriander, turmeric, cumin and curry powder (with optional paprika); the Zaal uses curry powder, coriander, cumin, turmeric, chilli and garam masala; Abdul Mohed uses curry powder, paprika, turmeric, coriander, cumin, garam masala, tandoori powder; Salvador Dali uses mild Madras curry powder, turmeric, cumin, coriander, paprika, garam masala, ground fenugreek and garlic powder; and so on.
Also this quote from Curryhell is highly significant :
Bear in mind that after the intial grind Az wasn't happy with the cardamon overpowering the mix and added additonal cassia bark and Asian bay so you may want to add another stick of cassia and 2 or 3 extra bay leaves. The powder should be darkish brown and the smell of cardamon should just be detectable whereas the cassia and bay should be fairly prominent.
In other words, while the amateur chef may make his (or her) "mix(ed) powder" by formula, the seasoned professional adjusts the quantities on the fly to achieve his/her desired goal ...
** Phil.