Each player's physiology is different - length of fingers/thumb, flexibility, etc. There may, or may not, be a 'standard' way to position, but basically, it's going to be down to individuality, and each player will find his own best method, which will change as the chord shapes change, or when playing only one or two strings. Or when seated or standing.Or how high or low one has the uke. It's pretty well much the same syndrome with guitar playing - there are no right or wrong ways - as long as the product sounds good - just because I put my hand in a particular way doesn't mean my students must do the likewise. Whatever works best for the individual is probably the best answer!