lactose should be converted 1:1 to lactic acid, so you need to figure out how acidic you want your BW, also you need to remember that lactic bacteria will stop working at less than 1%v/v lactic acid (very very sour) in addition to the alcohol and IBU's working against them
Id say, add maybe a 0.25oz or less in a gallon and see how it goes, if it gets too sour you can blend, if not sour enough, either add more Lactose to the gallon, or add more beer + lactose