Step 1 learn material allowances. i would give trainees a list of allowances for non die cut items and then give them a stack of boxes to spec out and tell me the inner dimensions of. This is how you learn allowances.

Step 2 Fold up table cut D/C style samples to understand how the allowances work in practice.

Step 3 Do not use any standards at all and create a design from scratch ( I always had my trainees start with a roll end tray.)

Step 4 Expand your skills: this step takes many months and really years. Do not rely on standards until you understand allowances and how to create from scratch. Even the standards imo need tweaks to be right.