Step down converter design calculation
Is my approach for power dissipation calculation and junction
  temperature calculation correct for the step down converter used?


No. Your approach to power dissipation is impractical and cannot be pre-calculated.

The practical approach would be just to take the rated efficiency curves (80%-95% efficiency, whatever), and assume that the remaining power is dissipated in IC and inductor. This assumes that your layout is done in full accord with manufacturer's suggestions, and the critical components (inductor, caps ESR) are used by their recommendation as well, so the advertised efficiency is achieved.



More accurate way is to use the Texas Instruments WEBENCH design tool, which will calculate all power dissipation for you based on recommended components. You will get ripples, ESR, etc.