You'll just "know" when you have the right layout - it will "feel right".BUT, if at any point during the top construction you have a niggling doubt about a block: FIX IT THEN AND THERE. Don't think if you leave it you will not notice in future .... you can guess how I know this!
I often think the first layout is the right one, but as Bilbo says, any worries, fix it at this point. I think we've all been down the route of thinking "it'll be alright on the night", but it never is.
PS Sorry, I also meant to say that it really does look lovely whichever layout you opt for.
Yep - happy now, thank you, both. Good point about fixing niggles as you go, Bilbo, whether layout or matching seams - I try to limit the amount of time I spend in any one session (no matter how much I want to press on) as it's when your tired you think 'oh, it'll be alright, really' - and it won't be when you come back to it!