I've found a good "practical" rule of thumb to be: Find the furthest location from where you started, there will be a AI homeworld at that location or any other location within 3 jumps-distance away (ie: if the furthest place is 14 jumps out, the AI can be on any world 11 jumps out).
Also, if you're looking for AI homeworlds, remember that every adjacent world to an AI homeworld will be tech level 4, and have a large number of special structures.