It seems like a lot of folks on reddit have questions about getting their first layer height properly calibrated. A big part of the problem I think is that the prusa first layer file only has one little square at one location.
So I made this larger version that has nine squares, spaced across the important corners. I've included prebuilt gcode for PLA and PETG.
This file helps for two reasons:
I recommend as it lays down the single line around the bed that you watch how that line is sticking in the corners, if it is pulling up you probably need to lower live z more (or use bed level calibration to fix just that side).
Also - someone on reddit made a very nice video so you can see what "good" first layer looks like.
NOTE: I included gcode to make this easy for "new users" (similar to the file provided by prusa). But later after you have your extrusion multiplier & ideal temperature for "your" filament you might want to come back and slice this stl with your slic3r config.