I've never heard of a bodyweight vs. pannier weight ratio but judging from the plethora of questions, you've asked, you strike one as a person who is super conscientious....and maybe easily discouraged if things don't happen as expected = it's better you cycle with less weight than your body weight.Is there a ratio formula of gear/pannier:body weight:bike weight? For example: If I weigh 120 lbs (54 kilos), my personal possession (not including pannier weight,) should not exceed X number of pounds?
I weigh 98-100 lbs. I think I might have cycled at most 40-45 lbs. on my bike for 2-3 wk. camping and cycling trips. But over the years I've gotten smarter and smarter and really reduce the amount of clothing.
Before you go on such a trip, you need to seriously take longer trip rides of at least 2-3 or 4 days back in your home province /state. Simpply ride longer distances every month locally, do it nearly every day now in snowy weather.
I would also on a regular basis every week, cycle with pannier weight by going shopping and build in extra long distances before getting home.
Netherlands is flat but I understand of course, windy.
Be light as you can be with layered clothing. You will want to cycle-carry some food with you also during the trips.
Cycling shoes: at least mountain bike like cycling shoes. You will be able to push harder and with the weight, you will thank yourself.
Water filter ...well, if you are drinking from tapwater.... What do your Dutch friends recommend? I wouldn't be drinking out of their streams and rivers. I would buy bottled water. Remember, Europe is not quite like North America. North America has real wilderness, wild large, dangerous animals, lots of wild fish in rivers. Alot less left in Europe now.