One of the challenges of project management is making accurate predictions. But the second challenge is that executives often don’t fully understand project management and very often want delivery faster than your initial plan, regardless of how accurate that initial plan is. If you present an initial plan to a certain kind of executive, they’ll ask for faster or cheaper delivery (or both). So if that’s the case it doesn’t matter how accurate your initial plan is – in fact accuracy is a hindrance. What matters is how much padding you have.
It’s common practice to add buffer to plan for schedule slippage. It might also be worth adding some buffer for that executive review, so you can subsequently over-deliver in accordance with his or her expectations. Not optimal, but a perhaps a better option than having to try and find cuts that simply won’t be there in a completely accurate and well resourced plan.