Proper pricing?

I recently gained a client who has two dogs. One is an actual dog, and the other is a puppy. When I am pricing out a walk for both dogs, I charge $13 per walk, $9 for an additional dog, and $10 for a puppy rate. I guess I'm asking how do I ensure I'm not overcharging / undercompensating myself? For 3 walks a week, do I charge it as ( $13 + additional dog + puppy rate?) Any math help would be greatly appreciated!