Manually calibrating a Rostock

Hello,

I'm building a customized Rostock. I've obtained a number of contact points by lowering the print head (I've replaced the nozzle with a Sharpie) to where it just touches the bed. The current configuration suffers from a bit of "bowling" where the print head moves in an upside-down bowl shape instead of printing a flat spiral. To get these points, I move the print head to a predetermined (x,y) location, adjust the Z until the marker touches the glass plate, and then call M114 (Get Location) to obtain the point where the printer *thinks* the print head is.

For instance, here is a sampling of contact points where y=0:

(-80, 0, 2.0)
(-60, 0, 1.7)
(-40, 0, 1.5)
(-20, 0, 1.4)
(  0, 0, 1.4)
( 20, 0, 1.4)
( 40, 0, 1.6)
( 60, 0, 1.8)
( 80, 0, 2.1)

These results are for the following configuration parameters:

DELTA_DIAGONAL_ROD = 248.0
DELTA_SMOOTH_ROD_OFFSET = 168.0
DELTA_EFFECTOR_OFFSET = 33.0
DELTA_CARRIAGE_OFFSET = 24.0

Again, these points are where the print head *thinks* it is hitting the print bed. Ideally they would all have the same Z value. Instead, they appear to form somewhat of a parabola. (Wolfram Alpha plot)

Given this information (actually a much larger set of points covering the whole print bed) how would I calculate how much to adjust my parameters by, and in what direction?