What number of calories are ideal for an woman (80) with a feeding tube?

My 80 year old Mom has esophagial cancer, severe structure that doesn't allow her to eat much by mouth. She just got a feeding tube into her small intestine. Her original weight a year ago was 150lbs. She dropped to 136 in July 2014 (time when she was diagnosed with cancer and started having trouble eating). She is now about 121. She is being feed by tube and is currently getting about 1,200 calories by day. Heart rate elevated above 100 at times and blood pressure drops, suggesting dehydration. What would recommed should be her ideal caloric intake daily? She is currently mostly bed ridden since she says she has very little energy. Before she was diagnosed she would walk for miles up and down hills daily. She was independent, drove, did not have any health issues, was not on any medication, never drank, nor smoked. Any advice would be greatly welcomed.