Most of it is just trial and error and some of it is from talking to folks who have competed and had success.
The amount of carbs is pretty much identical to what I cut on. Only difference is massive carb load over 3 days. After 20 weeks of dieting, I'm mega depleted and can probably cram a ton of carbs in before risking spill over. And even if I do spill a bit, I would rather come in a bit soft than come in small.