Take some shots both indoors and outdoors and bracket them. You should be able to see quickly what the ideal speed is for you. I personally shoot Portra at 2/3 to 1 stop less than box speed (200 or 250 instead of 400). It works well for me both manually metering, and using the matrix meter in my Nikon (which seems to be setup form for slides - preserves highlights at the expense of shadow detail).

My guess is you are a bit under exposed, and the scanning isn't the best it could be. An optical print will tell you how well the film is developed.