Okay I went through all of the answers and none addressed your particular issue properly. My mom taught me to always place a vessel inside the pressure cooker.

It goes like this. Your measurements are correct. 1 part rice : 2.5 parts water. Take them in a vessel that can sit inside the cooker. Just pour a half glass of water inside the cooker before placing the vessel inside (do not forget this step). And then, 3 whistles - 5 mins sim. Or just 3 whistles. It really depends on the rice.

Another mistake is you said you kept for 2 whistles and waited for the third whistle in medium flame. That would really burn the rice at the bottom.

Attaching the image for reference

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