I'm posting my configuration to hopefully remove some confusion / be helpful. This configuration has been ground tested and flown with ArduPlane 3.8.2.
Note: The sensor is connected directly to the I2C port labeled I2C 2 on the carrier board.
Be very careful when changing the ARSPD_OFFSET and ARSPD_RATIO parameters. The values posed above work well for me but use caution when changing yours. Your sensor may require different values and getting them wrong can result in a stall / crash.