OK - the docs say:
Setting up a Wraparound Universe
To enable the wraparound universe, set AllowWraparoundMap to Yes. In addition, you have to define the wraparound rectangle using the WraparoundRectangle config option. The wraparound rectangle confines all map objects (ships, planets, minefields, etc.). Any ship, for example, that exits the wraparound region reappears within the wraparound region on the other side.
Note that unlike ships and minefields, planets outside of the wraparound region are not remapped onto the region. If a planet is outside of the wraparound region, then PHost pretends that the planet does not exist. This allows custom maps with fewer than 500 planets to be used. Just place the planets outside the wraparound rectangle. Usual map editing tools place such planets at coordinates like Y=0 or Y=9000 which is just fine.
The WraparoundRectangle config option is an array of 4 values. The first 2 values define the X and Y co-ordinates (respectively) of the lower-left corner of wraparound region. The last 2 values define the X and Y co-ordinates of the upper-right corner of the wraparound region.
Note that, since PHost 3.3c, the extreme top and right co-ordinates are no longer part of the map. In older versions they were, which could lead to the strange situation that a ship that moves from Y=2999 one ly to the north would end up at Y=3000, while one that moves 2 ly to the north and back one ly to the south would land at Y=1000.
WraparoundRectangle = 1500,1000,2500,3000
defines a universe where possible X co-ordinates range from 1500 to 2499, and possible Y co-ordinates range from 1000 to 2999.
Here is another common setting:
WraparoundRectangle = 1000,1000,3000,3000
This provides a standard-sized map which is seamlessly wrapped.
So - it looks like there is no seam. One ly off the map is one ly into the opposite side. Sound right?