Really don't understand this as using max on the denominator effectively you will always minimise the size of error as opposed to using a consistent reference. Then dependent on how you calculate a weighted error you are not comparing apples with apples by sku. A simpler way would be to always use either forecast or actuals, absolute the error (I.e ensure it is positive) and then do a max of 0 or the output. Much easier to explain that there is a point where the error is so large it's pointless trying to scale it (in this example the error is as big as the original denominator). Equally when displaying bias you need the same denominator at all times otherwise how do you know direction of the error.