I'd like to ask how weight borne by shoulder straps should be distributed (between the top, front and back of the straps)?
Honestly, I just don't see how any part of the shoulder strap but the top can bear any weight.
Assuming the hiker is standing erect on level ground, the pack is exerting force straight down. This force is counteracted by support from the parts of the body relatively perpendicular to it....the tops of the hips and the tops of the shoulders. Look at my AV.
Imagine a theoretical pack with no hip belt and straps that only go as far as the chest in front and the back of the shoulders in the rear. What would hold this pack up? Maybe a little friction but that's all.
Tightening the shoulder straps may make you feel like the chest is bearing some weight (maybe a small amount by friction) but in my experience it is actually borne by the hips and tops of the shoulders. What tightening the shouldr straps really does is take weight off of the hips and transfer it to the tops of the shoulders. Conversely, tightening the hip belt takes weight off of the shoulders. I feel that is the key to your problem.
The percentage of weight borne by the hip belt is a matter of personal preference. Personally I prefer to carry most of a moderate load on my hips and keep my shoulder straps very loose. This allows air circulation between the pack and my back.
If there is a gap between the top of your shoulder and the shoulder straps, your pack may be too big for you, the shoulder straps are adjusted too high, or you are wearing the hip belt too high.
Could you post a picture?