I agree, give her the same volume as her regular food.  Maybe a little more if she seems hungry.  That is what I do when my boys are on the fresh chicken and rice.  They normally have 1/4 cup of their kibble, so I give them a little more than 1/4 cup of the cooked food plus little chicken bit rewards here and there.   
I would discuss the food with your vet.  Ask what she/he recommends for a heart healthy diet.  The fat content and general richness of the pouches and the RC might be too much.  It could be a factor in her gastro problems too.  My Teddy can be sensitive that way.     
Best wishes for your Lucy.     
				__________________  Kristin, Max and Teddy          |