First off you have to have an understanding some of what goes into making a map for each game

From my Research a map is made based on 3 items when using a .DEF file to make a game.
1. the Random seed used in the .DEF File. This give you one of 4096 possible maps as long as items 2 & 3 stay the same. I've ran a few million gens and if items 2 & 3 don't change you get the same map every 4096th gen (so random seed X & X+4096 are the same)

2. Each major race type is assigned some unknown value and these are added up to get a second seed. You can change the order of the races and you get the same seed, if you change any one race then you get a different seed. So if item 1 & 3 stay the same and you switch one race from SS to SD you get a different seed sometimes.

3. Within each race a value is assigned by the settings in the Custom Race Wizard and these are added up to get a third seed. If you change any race settings you may or may not get a new seed as I didn't research every setting but changing a races Habs or growth rate will change the seed it appears. So with 6 races in a game the 6 values are added together for the seed.

There may be more to it, but that's what I've been able to figure out so far. Now these 3 items are put together and somehow Stars makes a new game.

Now if you know ITEM 2, the races used in a game, you can find ITEM 1 by genning 4096 games with those race types and find the one that has the same planet X & Y locations. But without ITEM 3 the home worlds(HW) will be different and all the planet habs also are different