[Exim] Exim Variables

I am trying to deliver using the maildir format. But I have a /home dir
structure like this

/home/a/auser
/home/b/bperson
/home/c/cguy
is there a variable I can use to do this delivery..

my local_delivery transport is like this
local_delivery:
driver = appendfile
create_directory = true
directory_mode = 70
directory = /home/${local_part_first_letter}/${local_part}/
maildir_format

Of course there is no ${local_part_first_letter}, but is there a way to do
this?
Thanks,
Joe Kezar