Basim, I can't remember Simba's age or weight, but I would just go by the directions on the dog food. Different foods have different measurements. For instance, I use Flint River Ranch, and my dogs get less of it than they did of Iams or Pedigree. Next time Simba goes to the vet, ask him if Simba is too fat. I can tell that mine are overweight right now. Its been so cold this winter that they haven't had enough exercise.
By the way, I think Simba looks great! If you can feel his ribs without squeezing his back, he's probably fine, but ask to be sure. I'll bet your vet would tell you if he thought Simba needed to diet.
Good luck!