I’m not the most qualified to respond to this, as a lot of this is outside my day-to-day, but the units of measurement depend on context. For intra-stave quantities that also have to take into account the current stave size, we have a ‘Stave Unit’ type that is multiples of a space. This is a rational type, ie a fraction, so it can deal naturally with quantities like 1/4, 2/3, 143/1000, etc without loss of precision. For quantities that are inter-stave or related to page layout we use a unit that internally we call ‘Puns’ which is equivalent to 1pt. This is a floating point value (64-bit currently), so it should have as great precision as you’re likely to need.
In terms of the UI, I’m not sure what our current plan is as to how to present Pun quantities, but I expect that at some point we’ll allow the choice of points, cm or inches, as those can be converted to an from Puns unambiguously.