I've always been advised never to have the drum much more than half full in terms of dry volume.
As for washing powder, I've recently taken a leaf out of Ruth Goodman's book, she of 'Victorian Farm'. What cleans clothes is the action of water being forced through the fabric - no more. Add detergent and it serves to stiffen and weaken the fibres, hence you need to add fabric softener, causing further deterioration to the fabric. You also tend to need slightly hotter water to dissolve said detergents.
So now I don't use washing powder, except for the very dirtiest stuff and the nastiest stains. The stuff is still coming out clean and fresh smelling, and it's softer too. Cheaper as well.