Hi there Jeremy,
I'm currently using the theme linked in the first post and am having the same issue. I've tried the plugin you suggested but it's still returning negative values.
I found a line in one of the theme's files that I believe may be the culprit but I'm not entirely sure what needs to be done to get it to work correctly.
$content_width = $disable_sidebar == 'on' ? 'one-whole' : 'lap-and-up-two-thirds';
Commenting out that line appears to fix the issue with the negative values but disables the sidebar of the theme automatically on all posts. I understand this is most likely a problem with the theme and not Jetpack itself but any insight would be greatly appreciated.