Others are more knowledgeable than I here, but I've done my time under heavy rucks. By heavy I mean 60lbs +
The heavy weight should be as close to the spine as possible. e.g. plates upright against the frame of the ruck.
If the heavy weight is too low, it will pull on your shoulders, fatiguing you faster. Heavy weight too up high will make the ruck unstable, centre of gravity and all that jazz.
Keep the weight evenly distributed. There's not much worse than a lop sided, unbalanced ruck after about 5 miles...
Always use the waist straps (if you can), the hips are the body's strongest pivot point, let them take the weight, rather than your shoulders.
Generally speaking I'll put sleeping bag/spare clothes, in the bottom of my ruck, besides being the least important items that are not an urgent item to pull out, they bring the internal bottom level of the ruck up so that heavy items can be placed as close to the spine as possible and in the mid/slightly higher section of the ruck.
personally I always carry a poncho in the top flap of my ruck for the extra layers of waterproofing and for ease of extraction. Then everything is packed with the most urgently needed items in the most easily accessible parts of the ruck, i.e. outer pouches and internally as close to the top as possible.
Remember, it's going to hurt sooner or later, and you can always fit one more item in, whether you think so or not.
