Mendel90/RUMBA/Marlin - Z-Axis too slow, X, Y work great

Hi all -

Mendel90 printer
RUMBA controller w/A4988 Pololu drivers
Marlin firmware
Repetier software

I have calibrated motor moves with configuration.h in the Marlin firmware, and set the RUMBA microstepping jumpers under each of the A4988s to be at '000' (single step).

My calibration in configuration.h:

#define DEFAULT_AXIS_STEPS_PER_UNIT {78.7402*1.05,78.7402*1.05,200.0*8/3,760*1.1} // default steps per unit for ultimaker

X, Y and E all are moving as expected, pretty calibrated. Z is moving waaaay too slow. I should only have to tweak the 200.0 factor a bit, not increase it by a factor of 5 or 6, right?
It almost seems like my microstepping isn't right, even though the jumpers are set correctly.

Any thoughts on how to proceed here?

Thanks,
Rob

You have a mendel90 not a Ultimaker so why are you using this config

from [reprap.org]

#define DEFAULT_AXIS_STEPS_PER_UNIT {80, 80, 200 * 16 / 1.25, ((200 * 16 * 39.0)/(11.0 * 7.4 * 3.142)) / 0.94}
this assumes 16 micro stepping and 8mm threaded rod at 1.25mm /rev

200 *16/1.25 = 2560
200.0*8/3 = 533

The values for X ,Y and Z would not normally need adjustment