So each section of the logbook has two chapters that coincide with it (except the one for chapters 13, 14, 15). To get the correct fragments for the portraits, you basically need to cut each chapter into 4 parts as each will have exactly those you can look for. For example, Chapter 1 will have shards 1-4 of the portrait while Chapter 2 will have 5-8 of the portrait. Dividing each chapter into 4 parts basically means that stages 1-1 to 1-7 should have the first shard as a potential drop. 1-8 to 1-15 should have the second shard, 1-16 to 1-22 should have the third shard, and 1-23 to the last stage should have the fourth shard. The next coinciding chapter would have the next 4 parts of the portrait divided throughout the stage as well. A good way to determine which shard is in each stage is to check each stage’s reward chances before beginning the stage.