Since it was pointed out to me how to adjust z-level in simplify 3d: gcode tab and adjust z offset.
I've been thinking how to use that to avoid having to calibrate the print bed or to simplify the process when i actually need to do it.

I've got spirit level utility on my android tablet, had a play with that and it will let you set any situation as being 'flat'.

Ah ha !
Use spirit level to level bd and z-offset to adjust height, seems simple enough.

So i put the tablet on the guide rods of the printer and zeroed it.
then put that on the printbed and adjusted till it was the same as the rods.

Should have worked right ?
Nope. Was the most out of whack I've ever seen it.

So I recalibrated normally - this time remembering to heat the bed - yeah.

But even with the bed heated the two readings still don't tally.

It was a potentially good idea - but the bed is not aligned to the guide rods, which is why you need the levelling knobs :-)

Just thought I'd document this in case anyone else has the same idea, and remembers to heat the bed and uses abs and cooks their phone/tablet :-)

In fact heating up a phone or tablets lithium ion batteries to 110c, could well cause them to catch fire or explode - so don't do it !