When we fast, we up the salt, because our kidneys stop holding on to water, right? So, how many calories does our body need before that happends? Is there a magic number or percentage of body weight?
I think the calculation for a precise amount of calories is a difficult one, but an estimation of osmotic blood pressure, based on urinary volume, and assuming stable glucose levels could be done, and so, estimating how much salt should be intaked.
I don’t see much point in this though. These are all estimates and there certainly is the chance that the numbers will be off.
Trust your body. You should be able to feel the need for more salt.