Generally the rule of thumb is to make your door 1-1/4" wider than your door opening. If you are using "Grass" hinges they have a complete spec. chart for all of their hinges which is very helpful...anyway they used to. I don't need it anymore because I just know to make them a certain size since that is the only brand I will use. Welcome the world of cabinet making. I used to make my own doors, but for the last 15 years have been having them made by a company which only makes doors all day long and it is cheaper for me to use them than to make my own. However, I would never regret the experience of making my own raised panel doors, really makes you think a lot!! If you ever get the chance to tour a door shop take it...it is amazing how they can make a custom door in about 3 minutes. Best of luck.