You deleted one of your messages... the one I was trying to quote... and now changed ur original msg from "1.2 to 1.5mm" into "1.4mm".  Now I am more confused than ever.
Using my mad dictionary skillz I have learned: 
caliper: an instrument for measuring the distance between two points (often used in the plural).
I never owned one of those.  I'm such a n00b.
Does 1.4mm mean that all WASD O-rings are the same thickness?  And the one 40a O-ring deforms more that the other one so it only reduces travel by 0.2mm?  While the stiff 40a O-ring deforms less and therefore reduces travel by 0.4mm?
Someone who deleted their msg 

 made a good point about the O-rings being in the shape of a torus which means they would deform more easily than an O-ring cut out of a flat slab of rubber.  Drat!  I was thinking my custom made 10a extrathick O-rings would deform a lot while gently raising the force curve.  But having a flat surface on both sides will negate the softness somewhat.  I donno how much tho...  Darnit!