Mesh Bed Levelling and G80 in Gcode  

Mesh Bed Levelling and G80 in Gcode

I have done some fairly substantial calibration on my my machine and used this forum as a basis, but want to clarify that I must have read something wrongly... somewhere and also clarify the zoning of this.
My left rear heated bed is lower than the rest of the bed, to the point that using the Bed level on the control panel I had set it to 50 and the print was in fact still not right at the rear left corner, left side and also rear. So the first question, as I can only adjust Left or rear of both, would adjusting the left and also the rear adjust the far left corner? In effect, does the transition line move diagonally from the centre to the corners to set the zones for this.

My process of thinking here, was firstly print a 30mm square dead centre on the bed, as this is where PJR confirmed that the overall Z position is calculated from. Once I got that at the expected height, I then printed 9 off 30x30x0.6mm boxes, in all 9 Geometrical positions on the bed.
this showed that at dead centre, it was .60mm as expected, rear left was .74, left was .72, front left corner was .68, and the right side was consistently around .63. Based on what I read, I adjusted Bed level to have +10 for both left and rear, and printed again. Still off, I repeated this process until my left was set to 49, knowing 50 was as far as I could edit I think, I did not want to use it just in case. I could still see, in both the first layer, that appeared with tiny gaps in it, and the height was still out and had made little or no difference.
So....I started to used the G80 command, with L80, R25 and found that the first layer was in fact worse, and that the .6mm square was now .75-.76 at the read left position. Getting worse. I now changed the G80 to be L20 R10 and see what happened, the bed appears to print a much better first layer, and am about to test the measurements once they have cooled down.
I keep records of what I have done, and I have used this same setting from the control panel with little or no affect and never use both the commands at the same time.
So the next question, I have I misread something in that these measurements are used in positive to raise the relative position of the bed to the head? or does it move the head to the bed and thus should be negative to bring it closer, and secondly, why would the same settings, one on the machine and the other in the gcode, provide such different results.

Sorry in advance and thanks if you can help

Nick