Yes, the problem occurs after I updated the firmware.
To find the best z offset is by using the ‘expert mode’ while printing the first layer (the ‘expert mode’ only works while the printer is printing, not while it is in idle state). Since the problem of the z offset is at ‘0’ height on the first layer, you have to adjust the nozzle higher by clicking the ‘up arrow’ in the expert mode, and see the print results of the first layer while doing this. Before you rise the nozzle, you will see the first layer is very bad because it is at ‘0’ height. By raising the nozzle little by little, you will see the layer become better and better, and if you rise it further more, it becomes worse again ( the nozzle becomes to high, check YOUTUBE how the best z offset should be look-liked). So pick the number which is best, write it down, and put it in the printer preset in the CREALITY SLICER software.
The problem while the printer is doing the ‘purging line’, cannot be fixed. Yes it will scratch the bed a little bit, because it will purge at ‘0’ height. I’m not worry too much about this, since the scratch is at the border of the printing area, much worried about the nozzle. The only one who can fix this, is the manufacturer via new firmware. I believe it has to do with how the sensor pick the height while doing the auto level. After the firmware update, it seems the sensor becomes less sensitive or the nozzle move down too fast during the proses, resulting lower number.
One trick you can do to fix this, is by putting 3 or 4 layers of aluminum foil under the metal bed before auto leveling (it will make the bed in higher position during auto leveling–cheat the height using aluminum foil). After it finish, remove the aluminum foil, and do the test print (without adjusting the expert mode). If it still too low, put more aluminum foil, and do the auto leveling again. Be aware, if you do this, you have to disable ‘auto calibration’ while sending the file to the printer, otherwise, the printer will do a calibration again without the aluminum foil prior printing the model, resulting error height again.
I don’t do this, because it needs more works every time I change the metal bed. I have 3 spare beds with different thickness, using only 1 bed is a waste of time, since I have too wait for the bed to cool down before I can remove the model, and start another printing, By having more than 1 bed, I can remove the bed as soon as the printing is done, put it on my desk, and use another bed to start the printer again and let the printer recognize the thickness of the new bed by enabling the auto level every time I send the printing file.
Hope these will help.
. Besides, I just share what works for me and hope it will help anybody who don’t use fluidd and too afraid messing around with factory firmware……..like me 
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