Speaking of just trying things to see what happens, I get some unexpected results when I play with the Continental Shelves value.

Here it is with the default shelves set at 1000 ft below sea level. This one also has all of my edits and such.
Gryphii-5b-1000-shelves.jpg

Here it is with the shelves moved up to 500 ft below sea level:
Gryphii-6a-500-shelves.jpg

What I had expected was to possibly lose my hand-edits and for the shelves to compress inward toward the coastlines. What apparently did happen was that my edits were retained (the world would look much different back in its original form) and that the landmasses were raised up, bringing the coastlines closer to the shelves. Now, the shelves did move in very, very slightly, and there are spots in the flat areas underwater where it dropped into abysses. But overall the change in shelf outline is minimal.

This is just uninformed guesswork, but I would say that while we tend to think of the significant boundary for the landmasses in FT being at sealevel, it looks like it's actually at the shelves. I had been thinking of it as working from the top down elevation-wise, with it just putting in the shelves wherever it happened to reach -1000 ft. But it looks like it might be the opposite - building the shelves first at whatever depth is defined, and then going upward from there. I haven't built one yet, but I wonder if there would be any functional difference between shelves @ -500 ft and shelves @ -1000 ft and the water level set to -500?