For a target weight of 30 pounds (I'm going with the upper end here so your regimen will be less drastic), an inactive dog needs around 775 kcal per day. Of course this is just a rough figure not adjusted for individual need, but it's a starting point for you. This caloric amount includes any food, treats and extras you feed.
Example:
If you feed a food that has 350 kcal per cup, you could feed two cups per day and have a 75 kcal "reserve" for treats and extras. If the food has 430 kcal per cup, you'd feed 1 1/2 cups and would have 130 kcal left for treats and extras.
Make sure she gets her calories mostly from protein and fat and cut out carbs as far as possible. Most "senior" foods are "carb bombs" with far too little protein and fat content.
A grain-free food like Innova Evo (the lower fat variety) or Solid Gold "Barking at the Moon" would work well for this. If you have trouble finding a low carb dry food, you would be better off feeding canned or replacing at least part of the dry food you feed with canned food or fresh meat (either raw or cooked).
If your dog constantly seems hungry, you can add green beans to bulk up the portion size for a while and then slowly decrease the amount to let the dog adjust to less food volume. A decent fat content in the diet also helps to let the dog feel more satisfied. Most dogs like the taste of canned (well-rinsed) green beans and they add only a very small amount of calories per serving.