Mixed units: index by largest or smallest unit?

[sffc]

In a mixed unit such as "foot-and-inch", in what unit should the scalar value be indexed?

Should 5 feet 10 inches be:

1. 70 foot-and-inch (indexed in smallest unit)
2. 5.833 foot-and-inch (indexed in largest unit)

ICU internally does option 2, but I don't see that behavior discussed in UTS 35.

@macchiati @younies @ben-allen

[ben-allen]

I've been assuming option 2, largely because I've been thinking of precision as being tied to the larger unit. An argument in favor of option 1 might be the meter-and-centimeter mixed unit used for person-heights in some locales; when creating a Measure using meter-and-centimeter as unit, it is likely more natural to give (for example) 180 instead of 1.80 for value.