You can customize this in the Advanced/Custom menu of Jrummy16's Droid Overclock app.
So let's take a look at the ULV 1200 preset.
400 MHz, 31V
800 MHz, 49V
1000 MHz, 58V
1200 MHz, 71V
Now let's go to the Advanced/Custom menu and adjust it as we see fit.
Select Preset 1
Now the next section is set up so that you can customize frequency (MHz) and Voltage (V).
If you want to reduce the first slot to 300 MHz, then maybe something like this might be more appropriate:
300 MHz, 24V
600 MHz, 37V
900 MHz, 50V
1200 MHz, 60V
Be aware that 300 MHz would be 300000 and 1200 MHz would be 1200000.
This is my current overclock preset. Please keep in mind that all phones are different and your phone might not operate well at these settings. Tweak the voltages until you find the best combination for you.