The most important thing is to think through (and explicitly state!):
1. What do you want to find out? (--> what will be the meaning of the result? And how will this help me towards answering the core question of the client?)
2. What are the subcomponents of this target metric? And how are these components linked (e.g., by summing up? multiplying? other?)
3. Lay out how to quantify each component
4. Do the calculation and then aggregate
5. Check the result for plausibility and discuss what it means
One additional thing: CAGR and NPV are concepts you should understand, however you will NEVER be asked to do such calculations in an MBB case interview! The operations will be far more elementary than this. MBB firms are NOT interested in your calculation slkills, but in your conceptual strength and analytical rigor!