The first question is a great thing to ask your vet. I would assume the main thing is to keep it at a consistent time each day, however, you should ask.

There are some MANY places that sell veterinary prescriptions. Many are not reputable! I would see if your vet clinic will do "price matching" for you and give you the medication for the lowest advertised price you can find. Often veterinarians prefer to do this because they want clients to be receiving quality prescription products. This is much better than doing business with a company with whom you have no history.