I have been in a similar situation. It has taken me 3 months to find something which works. My main advice is every dog is different and every food is different in regards to quality and the amount you feed. I am now using one of the cheapest food I have tried.
I have tried Skinner, Wolf Worthy, James Welbeloved, Millies Wolfheart (Multiple) and Canagan
Rachael recommended Canagan which is a very very good quality food which slightly improved Mowglis stool but still not perfect. This confirms my point that every dog is different.
I then meet a dog walker who was a stockist for a food called Ancestral Canine (link below) which I gave a go. Within a couple of days his stools were firming up. He has now been on this 10 days and I need to reduce the amount slightly by around 10-20g to make it perfect I hope.
In regards to feeding amount, this also depends on the dog and the food. For instants. Wolf Worthy I would feed 180g a day over two meals but Ancestral Canine I feed 310g over two meals and with James Belbeloved it was closer to 400g. This seems to be down to the protein content more than anything.
Most people say change the food slowly but after trying so many food I just switch straight to a new food unless it is a 80:20 High Protein then do over a couple of days. Millies Wolfheart told me you change switch between dog food every 48 hours! Not sure I would like to do it that often but they should know what there on about.
Another thing I read on the internet and do when switching food is to add a table spoon of natural LIVE yogurt to the kibble for a couple of days as it helps the system adapt etc. (This Works)
OVERFEEDING IS ALSO THE BIGGEST PROBLEM as other have said. Take the minimum amount from the guidelines on the bag and then reduce it by 50-75g and if the stools are ok you can increase by 10g every couple of days until they become soft again. Allow a good 24-36 hours to see changes in stools after adjusting feeding amounts.
Hope this helps.
P.S. I have stopped ALL treats while trying to find the right food as you will never know what is causing the problems.