I tested it on the bed and made a few decisions.
I had worked on the basis of the hexagons on the top of the bed and a plain border or around 12 inches overhanging. At this point I decided I wanted two more hexagons in the width (from 14 to 16 coloured ones) so there was one row overhanging on each side, then a border of around 12 inches - so there will be good coverage.
I also decided to finish each edge with a row of full black hexagons, rather than the half-rows I had before.
I then made the appropriate additions to the four rows to establish the new scheme.

By testing the 16 row width on the bed as the length, I decided that I would make the length 14 coloured rows, plus the border. I want this to be a working quilt, so I think I now have the dimensions that will work for our bed.
This has enabled me to make more accurate calculations about the number of hexagons I need - and additional black fabric I will need to buy. The numbers of 3/4" hexagons stack up to 2087 black, 128 red centres, 128 purple centres, and 1428 other coloured.
I had already identified and cut nearly enough coloured pieces, so have topped these up to the requirement and organised them in plastic bags for each row. I have bought the black fabric I need, and will gradually cut and add this to the bags, so each bag will allow me to progress one coloured and one black row.
So - all set for production!