z-axis cog may or may not be anywhere near the center plates, depending on suspended hardware.
An example being, my Hexa, fully loaded, z-axis is 3/4's of an inch below the bottom plate.
So, what you need to do is to hang the acft vertically and find out where the cog for z-axis really is by moving the suspendion point until the acft hangs vertically. Measure that point to the center of your imu.