I have also been toying about building a "black Box" to monitor things.  Besides the fuel level monitor, outside temp, data from EMC2 and such.  I can program it but building the components is the challenging stuff and that is why it interests me.  As i feed the airplane voltage into my application, I divide the current by 1/3 so that I can feed my microchip ADC.  In my prototype, I have been using 1M resistors to do the division.  Does it matter on the size?  My thinking is, since I only want to voltage, the higher the better.

Bob K