The primary reason for giving sub-q's is to maintain hydration. Fluids introduced subcutaneously don't go directly into circulation -- first they filter slowly through the interstitial matrix into the lymphatics where they gradually work their way up to ducts where they enter the circulatory system more or less "as needed" (i.e. not under pressure). Thus sub-q's present little risk of volume overload as can be a concern with IV administration. However, in exceptional cases such as existing CHF, we might want to give "just enough and no more".
This discussion deals with "fine-tuning" doses for those cases. (Summary at the end)
The goal is to balance total daily water intake with output.
Intake includes oral (food and drink) and water generated by metabolic processes.
Trying to measure all factors would be impractical, for instance weighing sweat and losses through the breath. Even measuring urinary output accurately isn't generally possible, and calculating the moisture in food probably would be unnecessarily time-consuming. We developed a simplified system for Coco that served us well.
First assume that the sweat and insensible losses are probably insignificant in the order of things, that metabolic input probably approximates loss to new protoplasm, and that moisture in food roughly equates to moisture in feces. This narrows it down to two variables: water drunk versus urine. Sub-q's should supply the net difference.
Rather than measuring urine (hard to get all of it) and water (how much lost to evaporation from the bowl?), it's more practical to track changes in body weight. If it's assumed that a cat's body mass is about the same in the morning as in the evening and the same as it was the day before, then it follows that any fluctuations in weight are due either to eating, drinking, urination, or defecation.
What we did with Coco was to establish a nominal base weight and measure the sub-q's to maintain that weight when she was fully hydrated. (Digital scales for infants are ideal for this purpose and readily available at reasonable cost.) The starting base weight was determined relatively arbitrarily... we picked a time to weigh her about halfway between sub-q's when she was at an average hydration state, and neither immediately before nor after eating, urinating, or defecating.
We kept a log of weighings with notes indicating whether each was taken immediately before or after a treatment, before or after urination, meals, etc. We weighed Coco whenever she was due to urinate, and immediately after. (After a while, it became apparent that Coco would normally void about 1 1/2 oz. If she did so four times in a day, that represented 6 oz or about 150 ml.) We weighed her before and after each treatment. This confirmed the amount given, since as we all know the markings on the bags are impossible to read accurately. For simplicity, we used a conversion of 25ml per ounce of weight. This is a couple of grams off, but accurate enough to serve the purpose. Thus if she weighed 5 1/2 or 6 oz more after her treatment than before it confirmed that we had given her 150 ml. We also recorded the amount of each treatment. Having this information on paper helped track her progress. If she weighed a certain amount one morning after urinating and defecating but before her treatment, and the next morning she weighed 2 ounces more, she probably had retained 50ml of fluid. We might take this as an indication to reduce that treatment by 50ml. The reverse was also true.