I recomment turning off auto fill flash reduction and controlling flash exposure via straight Flash Exposure Compensation. When mixing flash and ambient, I usually shoot in manual or Av mode, and adjust the shutter speed (via Exposure Compensation when in Av mode) to control ambient light, and FEC to adjust flash fill. In Av mode, try starting with +1/3 EC and -1 FEC, and adjust from there to get the look you want.