First pic is the controller from my bike, second is from my wife's bike. The marks on the piece of card behind the shunt correspond to the extra solder I added, black for the first one and red for the second one (I put the card back in for the photos hence you see both sets of marks in both photos). As you can see there was already some solder on both shunts, a bit more on the first than on the second (and at opposite ends). I followed the 20% extra mantra for both, resulting in about 2mm more solder on first shunt and 3mm on the second one.
As I said, my soldering skills are 'iffy'... I have only ridden the bike once since, on only moderate gradients (I was not up for going on a detour in the p1$$1ng down rain just to find some hills TBH) but it does seem to hold speed a bit better going up hill. I will know more tomorrow when I have to go into town and back up a long drag plus a short sharp 20% gradient.