For a Nicholson type of bench, I am going to be a contrarian, and advise against
a lower shelf that runs the length of the bench.  All the point below are good,
but due to the tall aprons, reaching beyond the front edge of the shelf will be
a pain.  If this was my bench, I would probably build storage into the end or
ends of the bench where it is easy to access, and possibly build a cabinet on
castors that could roll under the apron for easy access and removal. If a
lengthwise shelf is really wanted, I would make it no more than 8” deep, and
place it right at the front. That should be more than enough storage  for the
tools you are using at any one time without having so much space that it becomes
the defacto home for lots of tools.  It also keeps the tools where they are easy
to grab.

David Sobel