People really need to chill about the double space. Two spaces isn’t wrong, per-se, though it’s also not necessarily right. A single em-space is closer, but use whatever you like to read. The half-em is too small for my preference, but look—when (if?!) your work is typeset, it won’t matter what you used, and I certainly don’t care what form of whitespace exists, unless it’s there for specific stylistic purposes.
Just like everything else here, adopt whatever convention you prefer, and when the interior design/typesetting is done everything will be adjusted accordingly. If you’re targeting an editor, just adapt your copy for their preference or whatever the style-du-jour may be. Just please don’t tell people that any particular way is “wrong”.
Remember, a manuscript is source code for a book. Care about content, not style. ^.^
-Fox
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