Calibration of Z axis in Simplify 3D
The steps per mm in firmware is the link to the hardware configuration/setup. Reducing by half must reduce Z advance by half. If your prints are twice as tall, you must have serious under-extrusion problems as when the printer is laying down molten filament for e.g. a 0.2 mm layer, in effect this is a 0.4 mm layer. If this is not the case, you should also calibrate the extruder.

Please re-calibrate your machine Z axis in firmware (most firmwares allow command M92 to set the steps per mm), not in Simplify3d, and then the extruder.