I suppose I should have been clearer that the narrative aim of the battle is to retreat in good order to have an army capable of redeploying elsewhere rather than rushing away in a panicked rout, so if fliers/striders chose to break and run it would be because they were abandoning their comrades and making it every man for himself. Also, you make a good point about the specific distances; they could use more playtesting. Since the number of units decides whether you're best fighting or fleeing, the idea was that the side with more units (i.e. the fleeing side) would be forced into the tremor radius while the smaller side chasing them would be better able to dodge the effects, but perhaps the side areas are too generous.