DiamondCore Tools Trimming Spinner From  £8.66

I bought all three sizes and just used two of them when trimming four bowls I’d thrown. One tip…. Once you’ve got the pot centred on the wheel, get a trimming tool and hold it in the centre as the wheel spins to create a small nobble in the centre. You can then see through the hole in the Trimming Spinner to locate it perfectly on the centre of rotation.

I recommend this highly

I saw a Trimming Spinner being used on Instagram and thought I need this! It helps stabilise your pot when trimming plus I’ve found it useful setting out equal decoration marks.

I haven't used the tool for turning yet, just as a guide for marking out a design around a pot. I guess there are only so many colours you can choose, but I found the darker colours quite difficult to tell apart.
The base of my pot was wider than the large disc so I needed to find the centre of my pot, line up the hole in the disc, and lay a small ruler across the disc to get my marks.
I'm not sure its helpful to have the numbers around the outer edge as if everything started at zero, then simple maths says some of the coloured lines should overlap... and they don't.

Easier than using a set square or protractor... and may also be useful to aide turning but I can't comment on that aspect.

So easy to hold with your finger whilst trimming and the division markers make it easy to mark out designs.