i have it in the front of the thread, somewhere
go lookie
it does vary for people, and greatly. if you want maximum benefit, which in your part of the country that's what i'd shoot for, expose at least whole arms and face, and *just* before they turn pink. not red. not hot pink.
just that very pale sun-kissed, not sun-burnt sort of pink. i can't see it on dh, and ds never turn anything but golden, but dd and I do show it.
i'm told that an average biracial kid with med. brown complexion has about a 25 SPF. it seems my ds has more than that, and my dd has about that. just gauging by how my skin reacts when it has sunscreen on it.