How full is full?

Being as this is my first year burning coal, I've been wondering how full on my stove is full.
I have an old Wards coal stove that I got from Craigslist. Here's some pictures of both the stove and interior. Those corrugated pieces extend up the interior sides past the loading door if that makes a difference.

Before anyone can say anything, the mat in front is fire retardant sold for in front of wood stoves, and covers a couple of inches of granite as well.
I also know that I need to clean some ash spillage from in front of the ash door. I just cleaned out the ashes and restoked, and then got to wondering how full is full while stoking.

You need a minimum depth of 8 or 9 inches for nut coal and more for stove. Beyond that it will depend upon your need to still have about half the bed burning at 12 hour tending time. That in turn will depend on how hot you are burning. For higher heat, more coal. The stove can take much more, but greater loads will require more care in avoiding puff backs. Drill down with a poker to burning coal after a large load to provide a path to ignition for the gasses ,plus use some over fire air as well as plenty of primary air. When blue flames are well established you can then button up for a long burn.