Amusing, but effective ....
I already had a derailleur hanger alignment tool (since they are invaluable, if only to check that shift issues are *not* a bent hanger).
I then bought one of these:
https://www.bicycleservicecentre.co.uk/product/bsctools-lever-height-adapter/199
... which is an adapter which threads into the steerer.
Whilst all the above methods above, especially the
@Smokin Joe one work, this is remarkably quick, easy and accurate.
For added points, the silver nut on top,with the hole through the shaft, allows a bit of string / cable / wire to be stretched to the rear of the saddle to also align the saddle perfectly with the steerer tube.
None of it's necessary, but it's very much nice to have!