Cooking of rice: Though the usual ratio of rice to water is 1:2, I find it works better when I add 1/2 cup less than the required qty. Here for 2.5 cups of rice, I used 4.5 cups of water instead of 5.
Also, once the rice is cooked, I spread it out on a big tray and cool it before layering. I find that it helps the rice from getting overcooked or mashed up during baking.
If you are cooking this for a party, you can make the prawns masala 1-2 days in advance and refrigerate.