I'm also using the Simple Diet like essbee. I will ocassionally eat a dinner that exceeds the 300 calorie limit, but when I do, I make sure that my two entrees do not exceed 600 calories. So if I have a 200 calorie lunch entree, my dinner entree could (in theory) have up to 400 calories.
In practice, my entrees have been averaging about 250 calories each. My highest calorie dinner entree contained 350 calories, but my lunch entree that day had only 190 calories.
I've been on nutrisystem several times, and I can vouch for the Simple Diet being quite similar (except the food is much better tasting, much cheaper, and you have tons more choices).
If budget is an issue (and even if it's not), it pays to check out several stores frozen dinner prices and selections. So far, in my area, Walmart carries the most variety and the best price average, but many stores have better sales (Walmart will price match if you take in the sales flyer), and every store carries a different selection.
I have a wheat sensitivity (I can tolerate traces of wheat, such as in soy sauce, but pasta or bread will trigger an itchy breakout), and finding no- or low wheat dinners can be a bit tricky, but it's getting easier and easier.
One of my favorite meals is LC ranchero beef, poured over a small sweet potato. On the Simple Diet black beans, potato and sweet potato do count as veggies, but you're told to be very careful with the high cal fruits and veggies and to stick to the lower cal choices most often, so I consider it a bit of a splurge.