Hi there, when the features I'm mapping appear in both Bing and King County imagery I tend to swap between them while I'm aligning things. Typically whichever one the existing objects in an area better align with I'll use for reference. As for a particular offset value, rather than use a set value I find that I have to shift the imagery bit by bit to match existing object alignments as I move further in one direction. Hope that helps, happy to discuss further if you like!