It’s worth noting here that the reason you’re seeing different answers in parts of the Derivatives readings is probably due multiplying a factor by large notional principals. Anytime you do this, there can be some significant rounding differences. The important thing here, as noted above, is that you understand the concepts enough to get close enough to the correct answer. You’ll find that exam calculation questions will almost always use “closest or nearest to” language, and assuming you performed the calculation properly the correct answer should be clear even with rounding differences.
That said, I tend to take all of my calculations out 5 decimal places.