Feel for the back of the rib cage to the tip of the nose. This is the length to insert the tube. Less than that will not get the liquid into the stomach, more than that may damage the stomach wall. You will often feel a slight resistance when the tube enters the stomach - this comes with practise. When you open the pups mouth, insert the tube on top of the middle of the tonge.
People ask about getting it into the lungs.
If there is any question, pull the tube out and reinsert it. I feel that when the puppy is really dehydrated and sick, one needs to be careful not to feed too much. When an adult dog has diarrhea your vet will often tell you to not feed them anything other than water for 24 hours. With pups I think we need to be careful. I make a judgement call and will tube 10 to 15 cc of Lactated Ringers 2 to 3 hours after a formula feeding, especially if I feel I still have a dehydration problem.