Simple question - I have a Swarovski 3-9x36m scope with the 4A reticle. Per their specs, the subtention from the crosshair down to the edge of the vertical post at 100 yards is 25.2" at 3x and 8.3" at 9x. Is there a special formula to calculate these distances for 4x thru 8x as well? I would assume a linear taper would be too damn simple . . . thanks.

They are linear.  In your case, 25.2"/3x = 75.6" per magnifcation power so...
@ 4x  = 18.9"
@5x = 15.1"
@6x  = 12.6"
@7x = 10.8"
@8x = 9.45"

Note that there is some rounding there and that if transfered to MOA you would need to correct to the 1.1"@100 yards conversion.  I think you will find that the biggest challenge will be finding exactly where the in between powers are and keeping the magnification ring there through shooting string.  Also confirm that the POI does not shift when you change magnification.

Thanks.