NR 1.2.3 Upbeats: confusing explanation of measurePosition

[Federico Bruni]

In NR 1.2.3, Upbeats:

"""
So \partial 8 becomes:

\time 3/4
\set Timing.measurePosition = #(ly:make-moment -1 8)
e8 | a4 c8 b c4 |

_The property measurePosition contains a rational number indicating how 
much of the measure has passed at this point_. Note that this is set to 
a negative number by the \partial command: i.e., \partial 4 is 
internally translated to -4, meaning “there is a quarter note left in 
the measure.”
"""


IMO the first sentence is a bit confusing.
What the number says actually is "how much of the measure is needed to 
complete the time signature", 1/8 in the example above. This implicitely 
says how much of the measure has passed at this point: 3/4 - 1/8 = 5/8

I'm nitpicking... but I think that sentence is not so straightforward.
That number does not indicate what's passed, rather it indicates what's 
missing and this in turn is used to calculate what's passed.

I'm missing something?
--
Federico