400mm sounds good! Did you go for a smaller pendant?

I think it depends on the style and atmosphere you want to create in the room. If the room is a formal, traditional one like your picture, then one large showy pendant would look great. However if it is a modern or contemporary room then two ( or more) smaller pendants would be a nice feature. It would also pay to think about how much light you are expecting from the fitting, especially since it is a cathedral ceiling. Do you need the ceiling lights for good general lighting or just for atmosphere. Do you have any photos of your room so we can see a bit more?

My last household purchase was new sliding doors to the outside from my dining room. This then led to the building of a deck, new railing on said deck and existing terrace, new blinds as the old window length curtains no longer did the trick, then the paving stone path to the deck from the garden gate, some new planting to accommodate that, AND outdoor lighting for what has now become an alternative entrance to the house! Oh, and painting as well. So I can liken my last purchase to a bit of a triffid - it kept growing! But SO worth it! It has added another room/space to my house, increased light and warmth, added value and created a more welcoming entrance than before. Love it!

Have you tried a custom lighting specialist?