We're experimenting with this feature and really want it to work. We will keep refining it going forward. One really needs two radios or some very clever math to determine which side of the door one is on.
This could be an added incentive for customers to purchase your hardware for both their front and back doors. Then they've got two radios.
In two-door installations, it may be possible to look at the signal strength as received at the back door to tell when the user is inside the house. That could get you out of the time-of-arrival calculation business.
Another possibility is a small, cheap IR sensor. If there's a person inside the house within ~10 feet of the door, it doesn't unlock automatically. The basic idea would be similar to the sensors on toilets and urinals in public restrooms.
In any case I'm glad to hear that my Tom Swift amulet is finally being made. Next up, the Triphibian Atomicar!