Or... You want to bring up the Unified Bed Leveling system that just got merged...
I went to a several Dollar stores. I have the worst piece of glass that $.99 can buy. You can see dips and peaks in it when you look across its surface.
I get perfect adhesion across the whole bed 100% of the time.
It just got merged into the Marlin main stream code base.
(That might be a reason to load up the a newer copy!)
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github.com]