Z Axis issues after leadscrew conversion

I have an issue that has me flummoxed.

I have a Maker Farm Pegasus 12 where I upgraded the M5 threaded rods with a 0.8mm pitch to M8 leadscrews with a 2mm pitch.

Using the Prusa calculator I determined that the zsteps/mm should be 1600 (200 steps, 1/16 microsteps, 2mm pitch). I then updated the Marlin 1.0.2 firmware for the appropriate setting from 4000 to 1600 as follows:
#define DEFAULT_AXIS_STEPS_PER_UNIT {80.19,80.19,1600,90}

Unfortunately, my z axis is out of wack. It is like at least 4x what it should be.

I checked my RAMPS board and MS1, MS2 and MS3 are all closed so that means that the microsteps should be 1/32 but I suspect somewhere else it is forcing it to 1/16.

Any thoughts on where else I might look? Other options would be to contact Colin from Maker Farm or just keep dialing down the zsteps/mm but it seems like it should be 1600.

Thanks in advance.

For what it's worth:

Or you could just calibrate the Z direction.

[reprap.org]

As far as I know on the RAMPS boards micro steps 1/32 or 1/16 depends on which stepper driver you are using. If using the DRV 8825 drivers it's 1/32, A4988 = 1/16.

But is you go thru the calculation procedure it won't matter which ones you have.

Your new leadscrews are most likely M8 4 start leadscrews (same as mine) so your steps will be 1600/4 = 400

Thanks for the responses and the link it is helpful.

I am using the the A4988 driver so that makes it 1/16 and my my M8 lead screws have a 2mm pitch so using the link above (thanks kd6hq) to apply the equation:
steps_per_mm = (motor_steps_per_rev * driver_microstep) / thread_pitch I get (200*16)/2=1600 which should be right but we know that doesn't work.

So this is where I leaned something today. Obewan mentioned a M8 4 start lead screw (which is what I have). What I did not realize is that 4 start means there are 4 threads running together so that a 2mm pitch is really 8mm so that I need (200*16)/8=400.

I will play with this setting to see how it works.

That's why you go thru the calibration process. The calculations that you are doing are just a starting point.
There are several things that can make the calculated value be incorrect. One of them is pressure that the
spring exits on the filament, another would be the actual diameter of the gear that the filament passes over
verses the designed diameter. So even though your calculations may be correct the calculated value may
be off but it is a good starting point.

Good luck with your prints.

So many fiddly bits. Well, 400 was the correct setting and now I know what 4 start means as well.

You can see the difference in the attached picture.

I appreciate all the help.