How to Allow Different Authors of Different Levels to Edit Posts

I have searched for this answer and found an answer that works, but it’s not quite the safest because this would allow any registered member (once promoted to at least 1) to edit all posts.

What I am seeking to do is allow different authors to edit (add information to) each others posts that are of different levels, specifically users below my admin level to add information to a post.

Below are the changed I made in my functions-post.php file.

Lines 358 – 359:
($author_data->user_level <= $post_author_data->user_level)
|| ($author_data->user_level < 10) )

Line 398:
if ( $user->user_level < $other->user_level || $user->user_level > 8 || $user->ID == $other->ID )

Ideally I’d like to limit it to a certain restriction number other than anyone less than 10 as is currently set with the above changes. I’d like to put a limit in it that an author would have be at least level 8 to do this.

Does anyone know how to do this? Thank you in advance for your help.