Fore to aft does play a role in the effective run time +caster.
For example, my 2004 E450 has a nose down of 1 degree. This reduces my effective run time +caster by 1 degree. My unit runs at LH +5.3 RH +5.5 caster. It was a nightmare when I was running at +3.3/+3.5 caster. Adding the +2.0 degrees of +caster made it a different driving experience.
And yes, tire inflation pressure also effects the runtime caster trail, less pressure results in better stability BUT nothing like the difference of adding +2 degrees caster to what ever you have now.