Regarding Carrot Halwa

You can test with condensed milk or thickened milk. I have tried with milk earlier and I don’t think that it was tastier than this version with milk powder. So I have forgotten about that recipe and cannot pinpoint the correct quantity of milk.
About the lemon rasam- each and every lime or lemon is different from one another. Some has more juice and some has little juice. The sour taste also will be different according to the type of lemon. So you have to add the juice correctly to get the exact taste.