This is something you'll have to decide by yourself, but answering these three questions may help:
- Where do you get your mail?
- Where do private delivery services (UPS, FEDEX, etc.) drop off packages?
- If you had very important guests, which door would you have them use?
If the answer to any of the above three questions is "the front door" I recommend you consider that your main door, or "mouth of chi."
Once you make a decision as to which is your main door, stick with it.
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As a bonus, below is an image from page 26 of the book Feng Shui Your Own Home on the main requirements of an ideal main entrance.