Stuck on a Brain-Teaser - any advice or possible approaches appreciated

I've been preparing for an assessment center for a management consulting firm - looking through Glassdoor, I found that in the past, one of their favorite brain teasers to ask has been "How much concrete would you need to construct a multi-storey 500-car park?" I'm not entirely sure what would be a good way to go about this, and the approach I'm taking currently is to assume a 5-floor solid concrete block, identify its volume, and take 15% of it to identify the amount of concrete vs empty space. Specifically:

I assume the dimensions of a car are 2.5X2m for ease. Each floor will have 100 cars (10 rows of 10 cars each). There is a 2-way access road between every 2 rows (total of 5 access roads, each access road being 5m wide). Additionally, there is a 0.5m layer of concrete between each floor, as well as another 2-way access road along the width. This gives me a total dimension of 25 X 50m for each floor. Adding another metre on each side as the concrete for the perimter gives me 27X52. Taking an average height of 3m per floor and 0.5 metres of concrete between each floor, this gives me a total height of 17m. Therefore, the total volume is 27X52X17 or around 24k m3. Taking 15% of this as the actual amount of concrete is around 3600m3, which is my answer.

However, a lot of this feels very guesstimate-y, and I'm wondering if perhaps there is a better approach I could take. I've drawn a blank searching this up on Google. Any advice would be greatly appreciated!