I've used this website before to get calorie counts for sushi rolls:
http://www.sushifaq.com/sushi-health/calories-in-sushi/ Based on the numbers for the avocado and cucumber rolls, it looks like you'd probably be pretty safe getting two 6-piece rolls of most vegetarian options and still be able to stay within your 400 calorie budget.
Also, it might seem kind of gross to some... but when I get sushi and I'm with people with whom I'm comfortable, I sometimes scrape some of the rice off the outside of the sushi roll. I just don't need or want that much rice with my rolls, and I figure every little bit helps keep the calories down. You might also be able to request that your rolls be made with less rice.
Enjoy your dinner out!