1st layer adhesion - uneven bed temps in rear corners?

[gdog 4]

Yesterday I was attempting to print multiple copies of a 600x600mm bushing plate.  I would normally have printed these one at a time in the center of the bed, but I felt like experimenting a bit with cura's "print one-at-a-time" setting.  Anyway, I laid them out in a diagonal to print them from the rear-left corner to the front right so that the gantry would clear the completed prints when printing the subsequent one.  This is really the first time I've printed anything in that back corner.  Anyway, every time I started the print there was an adhesion problem with the back corner of the print.  It's possible it is leveling related, but since I had my IR thermometer handy, I check the bed temp and there is a difference from the rear corners to the rest of the bed of about 20-30 degrees (the digital thermometer increments in 5-degrees, so not super precise, but clearly indicating that there is a temperature variation over the bed surface.

 

I am wondering if others have noticed / dealt with anything like this.  I read something about screws that might needs some countersinking if the glass bed isn't getting good contract with the heat plate, and I'm going to check that later today, but basically the question is whether a 20-30 degree temperature variation is something to worry about if I'm mostly printing PLA, and maybe rarely going to print with other stuff like PETG. 

 

First layer adhesion in the middle of the bed where I usually print has never been an issue for me, but and 600x600 is about the largest base I print, but I would like to be able to use the whole print surface if a bitter project comes up.

Edited February 20 by gdog