Like OP, I have both a Slim and a Fat Boy but I use a setting of 9 for both. Astra SPs or Feathers. After about a month with them, I believe the Feathers in the Fat Boy and the Astras in the Slim match best.
Had to say 5 but it is an average. Around a 3 - 4 on my Mergress XL and my Futur (they only go up to 5 anyway), and around a 7 on my Gillettes (Fatboy, Slim, Black Beauty, Aristocrat).
I shaved today with my Slim Adjustable set to 4 and was so happy with my first pass on my face I stopped right there! Did a second pass on the neck though.