I've always felt that optimal pacing in an 800 was that the second lap should be approximately 105% of your first lap time. This seems to hold true at almost all levels, even the world record.
For a 1:58.5, that would mean:
1x + 1.05x = 118.5
x=57.8(first lap)
1.05x = 60.7(second lap)
For A few world records:
Coe/Kipketer 1:41.73 with the 105% rule should have split 49.62/52.09. Their actual splits were - Coe (49.7/52.0) Kipketer (49.6/52.1).
Kipketer's 1:41.11 would get splits of 49.32/51.79 using the 105% rule. Actual splits, you guessed it- 49.3/51.8.
For Rudisha's, the 105% rule would have him splitting his 1:40.91 in 49.22/50.69. His actual splits were 49.28/50.63.
Just for fun, I looked up Brenda Martinez's time from Works in 2013. Her 1:57.91 should have split 57.51/60.40 using the 105%. Real splits: 58.06/59.85. Hers are a tad off, but she also comes at the 800 from more of a distance background.