Well, I just openned a 3850 bios, changed the vcore to 1.5v (not that I'm going to flash that though!!), saved it and the HEX value corresponding to 1.5 v (DC05) is right there in the edited bios next to clocks. So, you can enter whatever values you want and the program is going to save it into the bios. Just remember that you are not limited to the drop-down menu values and you can enter anything in the box.

Having said that, we all know that there are limits to the voltage you can set via bios. I think for HD3850, no matter what number you put there, it is not going to be more than 1.29 on the gpu and probably a similar albeit higher limit exists for HD3870 - That's why people start doing pencil mods, etc.