If you're at 1200 calories and have been eating around 1550-1600, and your recommended calories are as much as 1800, you still have around 400 to 600 calories to play around with, as far as I can see.
You can do a lot with 400 calories. Make dinner a vegetable-laden egg-white omelet. Add in a toasted English muffin with jelly. Top it off with an orange. Voila--400 calories. Another possibility is a Lean Cuisine meal of around 300 calories with a side of steamed or pan-seared vegetables (I love snow peas for this because they're so quick) to bring it up to 400. A vegetable stir-fry will also let you get away with under 400 calories, especially if you don't fill up the plate with rice.
If you go with the 600-calorie recommendation, you can eat a substantial dinner AND a snack. Add a cup of yogurt and a piece of fruit or some vegetables dipped in hummus or even a slice of cheese and a couple of slices of deli ham or turkey to the meals I mentioned and you're right there at 600.