To stay perfect weight, you need to take care of Arthur/John. That means three meals a day, sleep at night, and avoid Downes Ranch at all cost. This is what I normally eat each meal to keep a good balance:

Breakfast (Around 6 A.M.): Coffee, baked beans, and a bread roll

Lunch (11 A.M.- 1 P.M.): Venison and some canned food

Dinner (6-8 P.M.): Big game, pearson’s stew, or a saloon dish.

After eating dinner, go to sleep until the morning around 9’ish. Some other good ways to maintain a good weight is to eat something (canned foods, etc.) if you notice your cores are dropping. If you’re in a campsite and are fast-traveling, that can have an affect on your weight, so try and eat before doing so if you haven’t had a meal in a while. Unfortunately, for those in chapters 5-6, these tips will not apply to you, since I believe Arthur is always underweight with TB.