This may be the easiest part of the whole letter writing process... if you're participating in the Acasta's Mail Packet project, then addressing your letter is EASY, it would be addressed thusly:
Recipient's Rank and Name
While a 'return address' was occasionally added, it was not a universal thing like we know on the mail of today.
Here are a few examples from the period:
From the British Postal Museum & Archive
and the Bath Postal Museum
and some examples from letters that were part of the last Mail Packet addressed specifically to members of the Acasta: