Cept for those high spots

Yep, they float. Installing the flooring to it, tightens it all up and there is no movement. For a nail down, and not a glue down, use ¾" plywood and 1½" long fasteners.

Yes it will be just as solid as a complete fasten down, as the weight and the thick 1½" of flooring and subfloor is stout.

Ya, if your floating, it does not matter if the cement compound bonds to the cutback adhesive, or not. You could use dry sand. All you going for is a media that will fill the gap. Not bond.

You could use layers of compressed cardboard to fill the low spot, if you wish.

I talked to a flooring contractor this week to get a better idea of what it would cost if I hired someone to lay the floors. I'm in California. When we were discussing installation methods, he stated that California code specifies that when building a plywood subfloor, the plywood must be cut into 4' x 4' pieces and put down with 1/4" expansion gaps.
Before hearing this, I had signed onto the 16" x 8' strip technique described here.
I've googled California code and subfloors at least 20 different ways with no decent results.
Is anybody here familiar with California code as it relates to plywood subfloors? What have you done for a plywood subfloor in Caliornia?

Thank you.

The 16"x18" exceeds CA code and might cost extra.

Thanks.
What do you think about the 1/4" spacing?

In the process of removing a few pony walls, we created inch wide, fairly shallow divots where the sole plates were attached to the slab. What product do you recommend to fill these holes? Bear in mind that we will be utilizing the roofing felt, 6 mil poly, 3/4" plywood, 3/4" solid nail down installation technique described earlier in the thread. Also, there is cutback mastic residue on the floor which we are not going to remove.
Secondly, our slab is in great shape and flat in most places. You said we could use sand to fill any low spots. What product would you use?

Thanks again.