Hi Arthur,
No, that is incorrect. The green ToolWindowContainer has a default ControlSize of "200,200". Since you are using Inner Fill (i.e. no Workspace), then control sizes are proportions. So effectively the green ToolWindowContainer will get 2/6 of the width and the black ToolWindowContainer will get 4/6 of the width.
If I run your sample, the window is exactly 800 pixels wide, the green ToolWindowContainer is 256 pixels wide, and the black ToolWindowContainer is 513 pixels wide. Taking into account the window's border, the window's content (i.e. the DockSite) can be 784 wide. The DockSite has a 5 pixel padding all around, so we have 774 pixels for it's content (i.e. the ToolWindowContainers). Since you have two ToolWindowContainer's you will need 1 splitter, which is 5 pixels wide, giving us a total of 769 pixels to divide between the two ToolWindowContainers. So to calculate the width of the green ToolWindow we use 769 * 2/6 = 256.33333, which is rounded down to 256. The same is done for the black ToolWindowContainer where we use 769 * 4/6 = 512.66666, which is rounded up to 513.