Z height question

Quick summary.

Kossel, but no Z probe at all, the allen key was so inaccurate it was easier to level the bed manually. I may fit a sensor if I can find one that will work on glass, but at the moment I don't have one at all.

So, what is my problem?

I have set the home>0 at 231mm in the firmware, it's 1.1.5 and all uploaded, the printer prints ok.

However, the LCD Control>Z height shows -3.5mm.

In order to get the nozzle down to the correct height I have to change it to 6.00mm every time I want to print. 1.1.5 doesn't seem to have a 'Save to Memory' option?

Can anyone tell me how to get a permanent correct z height?? I need to either add or subtract 2.5mm somewhere, or add or subtract 6.00mm, but can't figure out where, or which way?

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My other issue is that if I stop a print, or let one finish, as soon as the hotend drops in temperature I get a 'Heating Failure' message on the LCD. This means switching the printer off, readjusting the Z height on the LCD (2.5mm is a lot of turns on that poor knob) and then starting the print again etc.

Is there a solution for this. It never did it on my older version of Marlin??