me ...
my boxes bolt or lock on , i camp , boxes stay on unless i need to sit on them.
i lay out everything i THINK i want to take .
get the panniers and load them . too much to go in ?
unload them , remove stuff i don't actually need .
reload them , sort out what i can do without .
repeat till you are not quite full.
i did round the coast of britain , a few years ago , i came back with a bag i never opened.
what i need for a weekend, is similar for what i need for a week , what i need for a month , is about the same as a week.
things have to do 2 things where possible. clothes can be washed , or thrown away/donated . take old t-shirts and socks , you can clean the bike with them when used , and bin them = more room =less weight.
merino base layers , last 3-4 days before needing washing , i wash them in the shower , they dry really quickly , and pack really small.
i carry a min of 2 lts of water , and snack bars . and a days worth of army rations , these can be heated or eaten cold .
bags ontop of panniers ? i don't do that , i used to , but when you are tired , you will catch your foot on them getting on/off the bike , plus they aren't secure from light fingers .
when bike is loaded , i go out for a couple of hours to get used to the weight , and change in handling.
don't for get to have a ride at night to get your lights in the right place.
when you get home , unpack , and see what you didn't use.
it takes a couple of trips to fine tune it .
heavy things at the bottom , daily stuff in the topbox. money /camera's/phone/glasses, in the tankbag ( magnetic) so you can whip it off quickly .