Unit Rise Rule. 7 3/8" - 7 3/4", vertically.

Walkline Rule. 1'-0" outside the inner arc in two dimension plan view, i.e., on the horizontal, that line at which unit run equals 10".

Tread (without nosing) depth, or Unit Run Rule (really a corollary to the Walkline Rule). Horizontally — 10" at the walkline of nonlinear stairs; 12" for straight stairs associated with nonlinear stairs.

http://www.beforethearchitect.com/DC-STRs.htm

It may be helpful to point out that the 12" tread depth is the maximum depth at the outer circumferance of the stair. The tread narrows continuously to a depth of 10" at a point 12" from the center of the stair. This means that, of the three pages left side by side, the left most page would extend beyond the tread about 3/4" to the front and back, less the amount of the page torn off and left on the pad. The middle page would pretty much match the tread depth at 11" and the right most page would fit just within the 12" tread.
At the point where someone would typically step, approximately the center of the tread width, the depth would be about 11", not 12". Thus, If you can jump outward about 11" and down about 16" you could have cleared over the last step and the Ransom Note.

Here is a photo of the stair posted on another thread. It shows 12 risers from the floor to the ceiling, which typically would be 8'-0", giving a rise of 8" per step (96"/12=8"):

http://www.webbsleuths.com/cgi-bin/dcf/dcboard.cgi?az=show_thread&om=87&forum=jbr&viewmode=all

The second and first floor plans show the spiral stair to be set in an alcove with the second floor/first floor ceiling extending to the stair:

http://braveheart.users4.50megs.com/ramsey/crimescn1.htm
http://braveheart.users4.50megs.com/ramsey/crimescn2.htm

The ceiling of the hall terminates at the stair- I don't see that we need to project any vanishing lines to it.
Here is a great site with tables of dimensions for ordering a spiral stair:

http://www.stairwaysinc.com/design.htm

The minimum circumferance per National Code is 60". This would include a 4" pole with 28" steps/treads. Dave thinks the circumference of the Ramsey stair is 72", or 34" treads-could be. The standard tread angle is 27 degrees. At a finished floor to finished floor height of 107-109", depending on whether they used 10" or 12" joist (possibly 10"as this was the add-on part of the house, maybe 12" to match older building) this would take 12-27 degree treads or 13 30 degree treads. This would make the depth of the tread in the middle about 8" to 9" and each rise from 8.25" to 9".

Therefore, Dave's estimate of 8.75" per rise seems very reasonable.

And if you check out the floor plan links above go to the bottom of the page and see the photos of the stairs. I think a reasonable person looking at those pictures will admit it wouldn't be normal to bend down to reach below one's feet to pick up something on the next tread below, that is, unless you normally squat first and then reach down. Skipping a step and then turning around seems real reasonable to me.