As someone has stated the Exploration ships only get loaded into the world at around 20km, so there is a limited area a single scan will cover. My own results show it's just over 20km sometimes.

So I did the maths to figure out if the effective scan radius is 20km, what is the bounding box that fits within that area. I other words, we don't mind scanning twice, but want to avoid missing anything that was within range, but didn't get scanned.

So the magic number is 28.284km.
If you travel in a straight line, and run the scan each time you cover that distance, you won't miss anything.
If you want to conduct a grid scan (like search and rescue) use that number for the grid size.

The maths if anyone wants it:
d = r + r
= 40km
x = squareroot of ( r(squared) / 2 )
= squareroot of ( 400 )
= 28.284km