Recommended Data Center Raised Floor Depth?

We will be building a new Data Center in the near future and I’m looking for thoughts on the best “depth” for a raised floor. We have 12″ in our current Data Center and I’ve read where the optimal is 40 – 42″. I personally think it depends on what you plan to put under the floor. We use Cable Trays under our floor with Trunk Line cabling for Copper and Fiber that goes to each rack. Overhead, we use Electrical Bus Bars for our electric needs. Obviously, we also use this space for our cold air plenum.

Thanks!
Bob