But it isn't just a conversion factor exactly for the reasons you state: a cup of flour will be different weight based on brand, how much it settled in the bag etc. Always have to deal with weights.
The best approach is to watch a video that clearly demonstrates how the product should look and feel at every point along the process, and do what you can to imitate that - even if it means leaving your scales and cups in the cupboard.
The technique used for measuring "1 cup" affects how much weight you get in 1 cup. This is in addition to the type of flour, clumping, how densely it was packed in the bag, etc.