How much do you feed?

I have two young cats trying to get over a bout of diarrhea, so I'm feeding them wet food exclusively, adding water to it, and leaving running water out (last week Pippin got dehydrated enough that it was necessary to take him in for subcutaneous fluids).  The canned food is 100 calories per 3oz can, and the cats are both just over six pounds each (except Pippin who lost a lot of weight on the dry food).  The way I understand it, they should be getting about two cans a day.  I have no desire to overfeed them because my last cat was overweight and that could have contributed to her (timely but anguished) death.

I have some dry food that's not great, but it's never given them problems so it's okay in a pinch--but I don't want to risk dehydrating them when it's hard enough to get them to drink as it is.  They always act like they're starving and cry for more even after an entire can, but Pippin will sometimes wander away before he's finished and with dry food they're content to leave it out.   So I figure they're just interested in eating more without needing to eat more, but I don't want to find out later that it was legitimate distress.

What do you think?  Am I feeding them enough, too much, etc.?  Thanks in advance!

6 lbs are tiny cats-do they look fat? I noticed you neglected to put their ages too. That is important.

My 1.5 year old is not overweight at all and is 10 pounds. I feed her 15-20 calories a pound to maintain her weight.  So that is 150-200 calories  day. I find the calories on the package material or the website.

Today she will eat 4 raw Primal turkey patties for a total of 180 calories. 

Tommorrow she will get a full can/6 oz of Weruva Cats in the kitchen Chicken Frikazee for 156 calories total.

I think 200 calories a day is perfect at this time.

Ah thank you!  Unfortunately I don't know their ages, just that they weren't kittens six months ago so my assumption right now is 1-2 years, since they're definitely still very young and playful.  They also seem to have unusually high-pitched voices to me (for grown cats), but that's subjective.  My vet wasn't really able to give me an estimate but did feed Pippin on kitten food when he was hospitalized.

They're definitely not fat, but they're medium hairs so Merry can look it sometimes.  Pippin is definitely thinner than he should be, so it reassures me that 150-200 calories is enough for a 10 pound cat since he needs to put on weight.  They were also having the bad diarrhea at the time they were weighed, so it's possible they're supposed to be heavier and will gain that back now.