I disagree with this. Thermal expansion of the hot end is why you are setting a gap with a piece of paper or a feeler gage. It's also unsafe to be mucking around leveling with a 450 degree hot end.

Quote Originally Posted by curious aardvark View Post
Also it helps t heat the extruder as well, as that is also subject to thermal expansion.

I find it easier to remove the filament before levelling, otherwise the minimal ooze can stick to your paper/card and cause minor issues.

So basically you level the bed under the same conditions and temperatures that you print with. You have to bear in mind that you are dealing with really small measurements.

Couple of things to bear in mind for first layer.

If there are areas where it does not print, then it's usually because the head is too close to the bed and the filament cannot squeeze out.
So lower the bed a 1/4 turn of the level knob at a time till it prints.
Also change the speed of the first layer print - in your slicer - to around 10-20% of the full print speed. The slower you print the better it will generally stick to the bed.

I find that on bigger prints, quite often half the print time is taken up with the first layer. But it's worth it as it sets a solid and secure base for the rest of the model to print on and you can consequently print the other layers that bit quicker.