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?

Have you tried a custom lighting specialist?

My partner and I are designing our new build and we are now up to lighting, in particular kitchen lighting. We are having non-exposed scissor trusses in the living area with a lowered flat ceiling (2.4m stud) over the kitchen. Do we need pendant lighting? I am torn. I feel they are dust and grease gatherers, but also a great visual feature. I have attached a picture of our plan and inspiration for the living area. I am extremely grateful for any advice!