Carb usage at rest

I've been designing a diet around the idea of eating the desired amount of protein, enough fat for Omega 3 and 6 requirements with some extra Mono and enough carbs to satisfy body usage without converting fat or protein.

It seems the fresh tasty brains requires about 135 grams of carbs.

From this site
https://www.bodyrecomposition.com/nutrition/how-many-carbohydrates-do-you-need.html/
It would seem every 2 sets requires 5 grams of carbs
It also implies that outside of serious cardio your carb usage is 0

From another site it seems carb use from cardio ranges from a bit below 50% to 75% within a heart rate of over 115+ for a 48 year old.

** My questions.**

1: I've read elsewhere that at rest you use carbs for 1/3 of energy, would that just be an estimate of the 135 grams mentioned of is there carb use for activity like sitting about or lying on the couch. If so, what sort of estimate should I use. If I use 1/3 of calories of the BMR and general activies I get 900 calories, or 225 grams.

2: I've been using net carbs, however it seems that fiber is a source of energy [2 calories vs 3.75], should I use net carbs, total carbs or perhaps net carbs plus half the fiber. I suppose using net carbs would provide a bit of a buffer in case my math is wrong.

3: I've been losing over a pound a day for the past few days. I figure before the diet I was probably pounding back 4K calories and am now at half that, so I can see losing half a pound, is the excess a sign of water lose from carb depletion or is it just a weight loss variant of noob gains.

I know all this seems kinda OCD, but I am more a nutrition geek... really. Promise.