They are all generic guidelines. Don't count on them too much.

What you need is a reproducible offset to get the same value everywhere in the bed, then you can correct the overall offset via gcode at the beginning of your print.

You can also get an approximate Z-offset value, then print a solid 30x30x5 mm cube.

If, starting from the third-fourth layer, you see some over-extrusion, you need to adjust the extrusion multiplier or the E steps calibration and reprint.

Once the solid cube looks good starting from the third-fourth layer up, then you can tune the Z offset by printing a cube which is only 0.2 mm high (or 0.25 mm, or whatever your first layer height is).

If you see over-extrusion, the Z offset is too big. If you see visible gaps between extrusion lines, the Z offset is too small.

If unsure, better get some small remaining gaps rather than overextrusion and excess material.

The idea is that extrusion issues in the first layer can be caused either by the offset or by the multiplier, so you must adjust the multiplier first by looking at the print after about 1 mm height (the first layer issues tend to disappear after some layers). Once you know the extrusion is right, set properly the first layer.

Edit

For a good guide check Andrew Ellis' guide: set first the extrusion multiplier and then the first layer squish.