String Function to Get Left 1000, But Only To Last Period or Carriage Return
Edit: This was posted to address an earlier version.

The left 1000 characters are easy.

Then find the first period in the reverse of the left 1000. That's how much you should subtract from the original 1000.

Like:

LEFT(@s, 
    1001-
    CHARINDEX('.', 
        REVERSE(LEFT(@s,1000))
    )
)


...I think - you may have to add one somewhere if it wipes the period or leaves a space.