I'm with catlad on the 600 c/c but I guess the theory still works with 400's. But it always seems to get messy around trimmers and double trimmers etc.
Read our post to catlad.
Regards trimmers, nine times out of ten you can still keep your joist centres round stair wells. On the tenth you slip in either one or two extra joists.
One other point. If you are the coddy, it is for you to tell trades what you want and not for trades to tell you what they will do.
Regards oldun