When you adjusted the corners to 0.2mm did you raise the nozzle to 0.2mm?

It's common to leave the nozzle at zero, after homing, then adjust for 0.2mm. When the nozzle is raised to e.g. 0.2mm for the first layer you would actually be 0.4mm from the bed.

It's not uncommon for a bed to differ 0.1mm between corners and centre. The solution is to print an adequately thick first layer to compensate. Cura defaults to 0.3mm and I tend to use 0.2mm for layer heights of 0.2mm and less.

Not sure what you mean about setting the nozzle height. I was following a youtube setup guide. Just adjusted the screws in the corners to get 0.2mm. Then found the nozzle was practically touching the bed in the centre so adjusted the four screws equally so that the nozzle in 0.2mm from the bed at the centre - this meant it is about 0.3mm at the corners. Then did a test print. Didn’t make any other adjustments. The guide made no mention of nozzle height so unsure where I would set that.

The test print worked fine.

Cheers,

Nigel

Your slicer program should have a setting for first layer height, either as a set number, or a percentage of your layer height for the print. From it's percieved z 0, it will raise by that amount. So, if your z (sensor) offset is .2mm, and the first layer height is .2mm, then the total offset would be .4mm (from sensor). If your nozzel was already .1mm above the plate when you calibrated, then it should place the nozzel .3mm above the plate (.5mm, with respect to the sensor). Sorry if this all seems confusing.

For the record, my plate was anything but level either. I actually avoid a particular section of my board, 'cause there's dent. Measured by a rolling needle caliper, my bed varies by at least .25mm over it's surface. Of course, it spent 6 weeks in shipping. It's hard to keep .1mm deformations off a thin board like that. And aluminum's pretty soft. And the tape I've been using is anywhere from .1 to .15mm thick, so meh. I'm moving to glass too, though. Be carfeful about nozzel crashing with glass if you use an inductive sensor.

A good solution is to add a glass plate to print on using binder clips to hold glass to metal heatbed glass seems to be flatter.....I use a 0.1 distance instead of 0.2

I have this problem with my Anet A8. After a lot constant adjusting and checking the flatness of the bed and the glass. I discovered that the problem is sagging of the X axis due to the weight of the extruder on the 8mm diameter rails. there is also some sag on the Y axis but to a lesser degree. Not sure what the answer to the problem is other than my next printer will have more rigid X and Y rails.