Center of plate finder.

[RegB]

When applying purple glue stick to the glass I initially covered the whole plate - which made no sense for ~5 mm parts.
The center thermistor hole of my V6 onyx was a rough guide to middle, but I soon made a heat spreader out of white aluminum "coil wrap" {used to wrap wood window sills when installing vinyl or aluminum siding}.

I could have drilled a very small hole in the center of that, but it was a simple matter to draw three lines across it; X tower center to fixing screw diametrically opposite it, Y tower center to..
Z tower center to.. etc. Two would have been enough, but the third provided confirmation.

I still had a problem with gluing enough area without going way outside the part, so I drew a series of concentric circles on it with a cheap plastic compass at 25mm intervals.

You could go to all sorts of extremes with this, e.g. a square with inscribed circle at several size increments.
I stopped at 5 circles.

[Jimustanguitar]

Cool idea. My heat spreader has a little center punch mark in the middle, so I've always used that.

You could always dry run the first layer to see how far out your loops are. It'd give you something to do while the bed warms up