[Alfred]

For me (and this is clearer to me because I gave uke lessons at lunch last night on the subject of rhythm strumming) the up and down correlate to the rhythm of the bit. Like counting numbers and the & when teaching strumming down on the number up on the &
Learn the rules then learn when to brake them kind of thing. For now I am still learning the rules for cross picking.

As far as plain old alternate picking, I think the rule is to end up heading toward the next string you intend to pick, while this doesn’t precisely apply to cross-picking, the rhythm theory seems to.

I could be wrong, I have been before and will be again.