The exposure generally looks OK, but you forgot to adjust for backlight and sidelight. The old rule of thumb (with manual cameras) was to open up about a stop for backlight and about half a stop for sidelight. Automatic exposure cameras handle these situations in a number of different ways, and you just have to figure out what your camera does and adjust accordingly. Portra 400 probably works best at box speed, although some people prefer to overexpose a bit. Just don't go too far. 320 is a perfectly acceptable speed for this film, and it gives you a bit less grain and a bit of room on the shadow side. Try bracketing and see what works best for you.