I mostly ride with just a rack top back (AGU Quoram 575). Sometimes I want to carry a little more. The bag is expandable to carry all I need but when full is not the best arrangement to get at things.
When I commute I travel with a Ortlieb front pannier on the back. This is still larger than I want.
All I want to carry. Left hand pannier rain coat. Right hand pannier legs, arms and extra layer. This leaves the top bag with the tools, tubes, suncream, brevet card etc.
I mostly ride with just a rack top back (AGU Quoram 575). Sometimes I want to carry a little more. The bag is expandable to carry all I need but when full is not the best arrangement to get at things.