12.6, I don't know. In an ideal world you would put exactly 12.0V on the reset pin, and use the transistor switch to get the timing right of exactly when the 12V is applied, and follow the data sheet exactly regarding the programming sequence. If you deviate from that, you are experimenting, and may learn that something else works most of the time or some of the time, or you may burn out an inexpensive component (like the processor). These AVR processors are pretty hardy, and it could be that even 20V is OK, especially since you are using a resistor. The closer to proper specifications you use, the more likely the process is to work. It's all about costs and risks and learning.