Flex-basis is not a guaranteed measurement because it depends on whether flex-grow or shrink are also in effect. so flex:1 0 400px would would be 400px if there was enough room but it may also be bigger because the grow will make it grow.
I think you’ve got the basics but it is quite complicated because they all interact with each other and you can have quite a few variations depending on content and property.
Also flex-basis will override a width that has been set but will obey min-width and max-width.