...add one spoon of the already mixed powder...
Without wishing to speak for Rich, RD, my reading of it that you prepare the Jaipuri spice mix by combining the spices in the ratio:
3 x garam masala, 3 x dried mint, 1 x cumin powder, 1 x fennel powder, 1 x turmeric powder, 1 x chilli powder.
1 tbsp of this Jaipuri spice mix goes into the pan per generous single potion serving.
...What is the ratio of cream and yoghurt...
I made it with a 50/50 mix and it worked well, albeit I doubled up on all the quantities to make a generous double portion. I used 5 large, fresh Spanish vine tomatoes which ASDA currently sell at