Calculate Ra

Hi, I am trying to calculate the Ra in the correct way. At the moment I use an external software which calculates it, but I want to do it by own calculation. I have a pointcloud, what was taken bij a sensor 500 points/mm. Points can be found at the link underneath. The columns are x, y, z, i, j, k. The external software gives me a 0.90 Ra. The traditional Ra method Ra=1/L × (abs Z) gets me an Ra of 1.3?
Could someone help me with this topic?

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[COLOR=#333333]The equations or the data actually read by the two methods are different..[/COLOR]

Simplify the data to not more than ten data points and compare the results. I'll bet Kelly is right about the data or equations used.