Well, honestly, if you can find beef chow mein for 90 calories a cup, then by all means, have two cups! I'm amazed. Chow mein is a noodle based dish. Beef, as a rule, tends to have more calories per ounce than chicken or shrimp. Is this not from a restaurant, but something you got at the grocery store?? Or better yet, are you making it yourself and have worked out the calories??
You know, you're counting calories. If you WANTED to, you could eat 1500 calories of just chocolate cake - bleh, you wouldn't WANT to, but you could.
What you eat is up to you.
But be sure you have the calories right...
EDIT: I'm personally very wary of some 'stated portion sizes' - I couldn't just have one cup of chow mein and call it a meal.
It's like saying 'one slice of pizza' from a frozen pizza... that's not enough to keep you going! So, 90 calories a cup? Yes, you need to eat more - just because the nutritional info is based on a cup, that doesn't make one cup, necessarily, a 'portion'. Make sense?