You never mentioned what's on top, a complete review of other possible HVAC runs, and
other piping might need to be considered. Ex. most modern house use rafters for branch runs, while old houses are feed from the bottom, if with crawl space. So ID'n everything in a this space and is IMO a wise application.
Buy a stud finder, while the client has some thought of the install, you don't want to
chew up timber to get it on the spot.
If your so inclinded; simple graph paper (which will help creat a scale) and stud
lay-out imposed with clear this up. You can also use opaque wrapping paper as an
over layer to light patten to a scaled kitchen. BTW you can have multi sheets layed
over each other. I bought onion paper (roll) which does the same thing, but it's pricie.
Blue painters tape is also a handy applicator.