Averaging not working

The following, for some reason gave me a -2.21 answer?

 case 'D':  // Show days remaining at average daily usage
      lcd.clear();
      if (PeakDepthIndex < 2) {
        lcd.print("Not enough data");
        break;
      }
      {
        int oldestDepthIndex = (PeakDepthIndex > DepthIndex) ? DepthIndex : 0;
        // Subtrace the current depth from the oldest depth
        float averageUsage = EEPROM.read(EEPROM_ADDR_DepthBuffer + oldestDepthIndex);
        averageUsage -= currentDepth;
        // Calculate the number of days since the oldest sample
        float days = (PeakDepthIndex + 1) / 2.0;
        // Calculate the average usage per day
        averageUsage /= days;
        // Display the current depth divided by the average daily usage
        lcd.print(currentDepth / averageUsage);
        delay (5000);
      }
      break;

Why don't you print out your intermediate values to see where your error is?

O.K, so the reason it was wrong was the ultrasound must of hit part of the pipe on the way down because 155 where it hit the pipe / 68 average = 2.27. Why the - symbol?

AWOL: Why don't you print out your intermediate values to see where your error is?

Yes >>why don't you print out your intermediate values to see where your error is?