Well, for one thing, as you know, all recipes are different and it's really hard to gauge what you are getting at a restaurant. What I would do is go with the higher number, the 409 because the object is to be higher when unsure. At least that's my method.
The thing is, over the long haul the numbers tend to balance out pretty well and a meal here and there that's off a little in the amount of calories isn't really going to make a difference in your weight loss goal, assuming that's your goal. If you were to make a steady diet of something then it would become more important to get a more accurate assessment of the truer caloric value.
But even then, no matter how steadfast you are about being as accurate as possible, it's still not an exact science. The main thing is to do the best you can, and as long as you're seeing the scale move in the right direction don't sweat the little details.
You also have the option of Customizing the mussels, then putting in a number which is halfway between the two values. You'd have to adjust the nutrients accordingly as well. This would be an option if you tend to eat this meal often.