Is there a proper way to score unplayed holes?

For example, me and my buddy went out for twilight, played 1-12 and decided it was too dark to finished.

I would assume I'd chalk this up as a practice round, but I was actually starting to do good so i'm just curious if there's a format to follow.

Here is what the USGA says:

Holes Not Played/Not Played Under The Rules of Golf
Q. How do I post a score if a hole is not played or not played under the principles of The Rules of Golf?"

A. For handicap purposes, the player must record a score of par plus any handicap strokes normally received for the holes not played or holes not played in accordance with The Rules of Golf. These scores should have an “X” preceding the number. For example, player A is not able to play holes 16, 17, and 18 due to darkness. Player A has a Course Handicap™ of 12 and holes 16, 17, 18 are a par 5, 3, 4, and are allocated as the number 4, 16, 10 handicap holes, respectively. Therefore, player A will record an x-6, x-3, x-5 on holes 16, 17, and 18, respectively.
Please visit Section 4-2 of the USGA Handicap System manual for further reference.

However, in this instance, you only played 12 holes. So you should record a 9-hole score and just let the extra 3 holes go. The USGA handicap manual states you need to play at least 13 holes before you go to the "holes not played" system described above.

Wow, that was an ultra fast and awesome response. Thanks!

i think the usga says it`s par plus any handicap shots.