Trouble using ADC6 on a pro mini

I've built my own board that uses a TQFP Atmega328p and I am using ADC6 as a battery voltage meter. It is hooked up to a simple voltage divider. I used an identical pinout on my board as is used on the Arduino Pro Mini - 3.3V 8MHZ version. My understanding is that the pro mini has A6 and A7 broken out on the board (well actually I know it does because I have one) and therefore selecting pro-mini as the board when programming should allow me to use A6. So far I have had no joy. The function that uses A6 is simply:

void check_battery()
{ //282 is the difference in value between 980 (4.2V full charge)
  //and 698 (3.0V empty charge).  We display the percent remaining.
  
  b = 0;
  for(int i = 0; i<10; i++)
  {
    b+=analogRead(A6);
    Serial.println(b);
    
  }
  b = b/10;
  Serial.println(b);
  b = (((b-698)*100)/282);
  Serial.println(b); 
}

I also tried using:

analogRead(6)

but it still didn't work. It just reads in a value of zero.

I notice in the boards.txt file that the arduino pro mini 328 3.3v 8mhz block shows the board variant as "standard". Should it instead be "eightanaloginputs"? I tried changing it in the .txt file to no effect. Not sure what else may need to be changed if anything.

pro328.name=Arduino Pro or Pro Mini (3.3V, 8 MHz) w/ ATmega328

pro328.upload.protocol=arduino
pro328.upload.maximum_size=30720
pro328.upload.speed=57600

pro328.bootloader.low_fuses=0xFF
pro328.bootloader.high_fuses=0xDA
pro328.bootloader.extended_fuses=0x05
pro328.bootloader.path=atmega
pro328.bootloader.file=ATmegaBOOT_168_atmega328_pro_8MHz.hex
pro328.bootloader.unlock_bits=0x3F
pro328.bootloader.lock_bits=0x0F

pro328.build.mcu=atmega328p
pro328.build.f_cpu=8000000L
pro328.build.core=arduino
pro328.build.variant=standard