I suspect he meant something like an improvement of 40 percentage points along the normalized 0-100% scale, so with a scale of −10 to 10, this would be adding 8 to their welfare: 8=40%*(10-(-10)).
(Or it could just be 40 percentage points along the normalized negative part of the welfare scale, so +4 on a scale of −10 to 10.)
I suspect he meant something like an improvement of 40 percentage points along the normalized 0-100% scale, so with a scale of −10 to 10, this would be adding 8 to their welfare: 8=40%*(10-(-10)).
(Or it could just be 40 percentage points along the normalized negative part of the welfare scale, so +4 on a scale of −10 to 10.)