The usual spread thin rule (reproduced below) is based on how many Planetary Empire tiles each opponent holds before the battle, such that the player with the larger number of tiles is at a points disadvantage (i.e. spread thin!) compared to the smaller empire.
There are two broad options:
(i) The players simply total the number of tiles that they have and consult the chart above.
(ii) Each player compares individually with their opponent.
(iii) Both of the above!
Here's an example:
The Tau hold 6 tiles and are being attacked by an unholy alliance of necrons and daemons. The necrons have 4 tiles and the daemons have 3.
Option (i) would see the tau get a bonus 50 points as they have 1 tile less than the necrons plus daemons in total.
Option (ii) would see the necrons get a bonus 50 points and the daemons a bonus 100 points.
We think we might go for option (iii), since it gives everyone a bonus, but would like to hear suggestions and narrative interpretations for any of the options (e.g. flank attacks on both sides for option ii).