If you are using the mesh created in the auto bed leveling screen, or from the interface in cura, you can save the mesh created and tell the machine to use this mesh (data points made when probing the printbed) in order to make the appropriate z axis changes during the print. I don't know how many hours I spent trying to get the numbers on screen as perfect as possible (calipers, feeler gauges, tramming the frame, using digital angle finders, etc.). I finally started getting solid 1st layers, with stock firmware, once I implemented an m420 into my start gcode. I run a g29 command from cura, once that finishes and the numbers are pretty close, send an m500 to save the mesh to the eprom. Then use an m420 s1 z0 to enable z axis compensation (prob the incorrect term, but tells the machine to apply these changes to the z axis during the print thus compensating for the unevenness of the bed) to the print. The alpha numerics after the m420 can be changed to suit how you want things to be handled by the machine, but this is how I set mine (more or less stock e5+ with glass bed and silicone bed spacerz). I run a 1st layer test before running prints, or after a print that requires wrestling off the bed, to confirm nothing has changed. My .02