I designed the video timings to match the Atari series because that was what I ultimately wanted to recreate. Their video chip was far more advanced than the 6845 and 6847 or any other video chip at the time. It could mix graphic and text modes in the "display list".

The video clock was based on the 14.318 MHz colour crystal (four times NTSC colour carrier).  This gives squarer pixels than the BBC Micro's 16 MHz timing chain. The ideal 14.75 MHz gives perfect square pixels but 14.318 is convenient for NTSC video and 16 MHz divides easily to the clocks used by the BBC micro chip set. The floppy drive controller needed exactly 8 MHz for example.

14.318 MHz is only 3% lower than ideal, while 16 MHz is 8.5% higher. With hindsight I might have used the commonly available 14.7456 MHz frequency used for baud rate generation. This is only 0.03% low.

I did not get round to making an ANTIC compatible model but I did add someone else's VHDL POKEY which worked as expected.