I will have to agree with the 19 hour 50/50 chance of spawning and then 100% spawning 12 hours later. A friend camped SF for 19 hours with no show. Then 12 hours later he poped and was taken down. I then picked up camp and waited ..at 19 hours he didnt show up so i waited another 12 and guess who showed up :)
1.) Find out what time SF was last killed.
2.) Wait 19 hours and pray he pops lol
3.) If he doesnt show at 19 wait another 12 hours, from my exp he will show 12 hours later.
Good luck and ITS worth it. :)