Okay, I have done some extensive calculations and base voltage is 0.6v (VDD1 OFF) and each increment after that is equal to 0.0125V. So 0x1e equates to 0.975v

So for Lehto's kernel, running at 0x48 is actually changing the voltage of the CPU to 1.5V (max that the voltage rail can supply in this case it appears)

Also, a slight correction to my list previously

VDD1 OPP1 0.975v
VDD1 OPP2 1.050v (Though stock N900 kernel seems to be running it at 1.075V)
VDD1 OPP3 1.200v
VDD1 OPP4 1.270v
VDD1 OPP5 1.350v

So Titan's ideal kernel is currently running between 0.975V (500MHz) to 1.2V (850Mhz)

CORRECTION: Maximum voltage the power rail can supply is 1.8V so in theory, you can push it to 0x60 (I doubt that the power supply would withstand that...though 0x50 should be okay @90% efficiency)