I'm not sure I understand the question. I use my fronts (on both front and rear racks) and never consider how full they are except that if I do not want the items to rattle. I roll down the top. My bags, last purchased 9 years ago, roll down well past the top of the backing plate. I put in an item of clothing or or backpack if I need more. For example, if one front pannier only has 10 apples and pears on a farmer's market trip, I include the light backpack I roam the market with.

This is entirely in consideration of the contents of my panniers. Ortleibs care not at all what or how you carry stuff as long as very sharp items do not contact the fabric. (I put a neat 1/2" slice in my first pannier carrying a very sharp, long kitchen knife. Contacted a local rafting company and they told me exactly how they repair a cut like that on a raft, what to get and where to get it. I now have a certified rafting repair and that 19 yo bag is as good as new.)

Ben