Not a hard rule but what I was thinking...I originally said 35 but that's 30 below ground, not 20. I wanted it 20 down. 35 is really close too close to lava too.
65 = Ground Level
45 = Subway GRID LEVEL. Preferably running along X/Z intervals of 25 to make a nice straight grid.
40 = Subway Level 2 (Run wherever dafuq you want)