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!
Began weight loss ~ January 18, 2011 at 280 lbs
Met original goal ~ Normal BMI (147 lbs) on March 8, 2012 Goal story and photos
Met revised goal ~ 135 lbs on May 12, 2012
Pregnancy May 2012 - January 2013
Post-preg weight loss ~ January 2013 - July 2014
Now maintaining < 135 lbs