Theoretically that's true but then you have to take into account insensible water loss (1-1.5L/day) and the volume and osmolality of urine (lots of dilute urine will raise Na, whereas in SIADH normal saline will decrease Na even though the serum Na is lower than the Na in NS).
When attempting to determine how much the Na will correct with a given fluid, it's best to use the Adrogue-Madias formula (not accurate for SIADH btw). One tip when using the formula is to repeat the calculation with each liter (if you want to know the correction after 2L of NS, do the equation twice - once for each liter...you'll see what I mean when you play with the equation).
Formula: Change in Serum Na = (Infusate Na - Serum Na)/(TBW+1)
If you REALLY want to delve into these topics further, I highly recommend reading Clinical Physiology of Acid-Base and Electrolyte Disorders by Burton Rose and Theodore Post. It's the most useful book I've probably ever read.
​
Also how do you think I got into derm? Im really fucking good at medicine. I read the entire Burton Rose's during intern year: https://www.amazon.com/Clinical-Physiology-Acid-Base-Electrolyte-Disorders/dp/0071346821
I crush at renal physiology. And if pheo is on your list w/ this hx and it's what youre telling the patient you think it is, you're proving my point.