One trick that saves a lot of time before you write any pattern: generate two saves that differ in exactly one known way (buy one item, then diff), and byte-diff them. The changed offsets are usually the field you care about, and the size of the changed run tells you the width. Doing this a few times gives you a rough field map without guessing at structure.

The other thing worth checking early is whether the file is compressed or checksummed. If entropy is flat and high across the whole file it is probably deflate or an encrypted blob, and hex staring will get you nowhere until you unwrap it. And if there is a 4 byte value near the header or footer that changes on every save even when the data is identical, treat it as a CRC or timestamp before assuming it is data. Writing back an edited file that fails the checksum is the classic reason a patched save silently refuses to load.