He talked about this in a podcast but I don't remember which one. He basically said the baseline carbs are the total carbs your body is able to hold as the water weight you lose the first the days is from carbohydrates stored. Keep in mind that this includes water held by fat cells so when you carb load it will be less since you only want to provide enough for muscle. I don't know a rule of thumb for how much to back load but my rule of thumb was no more than lean body mass based on body fat estimates. I don't even touch that since my main goal is more fat loss than muscle gain.