Hullo. Well, seems to me that what you should be looking for are Queries, not array formulae. Information scattered across 20 tables generally means that the data could be normalized. Not even going there, though.
My suggestion would be to start using the Query Design Wizard, and see what you come up with. It's not really that difficult once you start getting used to how Access displays things.
The Query Design is a graphical aide for writing SQL. For the most part, you'll never have to touch the SQL. Figure out how your tables relate to each other, and think about building some Relationships in your db. Whether or not you have Relationships, start a new Query, add the tables that you need, figure out here how your tables relate to each other, and drag down the fields you need. If you need to perform calculations, a Query is the place to do that. NEVER store a calculated field in a Table.
You can perform grouping, sorting, and filtering all with a well designed Query.
HTH
P