For every 1500 points of difference from the average, the average will change by 100 points. Notice that each time you make the additional ending score choice the difference between the normal high and normal low also decreases. Once you know all fifteen scores the high and low ranges will be the same, and will in fact be the Victory-point limit. That assumes no one lied to you about the choice of ending score. In general Pirates like low ending scores because they can gain the lead quickly, but they get smaller return on each world for multiple plunders so they slow up towards the end.
Berserkers are the opposite extreme.
They like high ending scores so they can have plenty of time to build and position their PBBs. In general a Pirate should automatically choose an ending score of 1,000 and the Berserker should automatically choose an ending score of 10,000. If they always did this it would be easy to plug in those choices for each player type, but not everyone chooses an ending score based only upon those considerations. Nonetheless if your game is top heavy in Pirates or Berserkers you may want to take that into consideration. I hope this tip might be helpful in determining your own choice for an ending score as well.