Truncating the_content

I am using a child theme of twenty fifteen.
I have this code in my content.php file

the_content( sprintf(
				__( '<br />Read more %s', 'twentyfifteen' ),
				the_title( '<span class="screen-reader-text">', '</span>', false )
			) );

This shows the full post content in both the post list and full post pages.
If the editor has used the ‘Insert Read more’ button a truncated version of the post shows on post list pages. This is all good, however…
If the editor has not used the ‘Insert Read more’ button I would like to be able to truncate the content to so many words and keep the Read more link working just as it is.
Is this possible?

I have tried using the excerpt instead, but then I lose my Read more link.

Thanks for any help!