So the 12 songs listed on the original cover would be about the right length
Yeah, approximately. I always make my mixes based on previous and post Beach Boy album lengths:
Summer Days (Side 1): 13:58
Summer Days (Side 2): 12:47
Party! (Side 1): 16:06
Party! (Side 2): 15:00
Pet Sounds (Side 1): 18:43
Pet Sounds (Side 2): 17:28
Smiley Smile (Side 1): 12:56
Smiley Smile (Side 2): 14:41
Notice how short Summer Days and Smiley Smile are. Since it seems that Pet Sounds was viewed as "special" in the boys' eyes it gets a bit more filled out time in there. I would think the same would go for Smile and so I try to make my mixes about 17-19 mins per side.