[css-transitions] Interpolation length, percentage, or calc

Hi,

I need help to interpret the following sentence on interpolation of length, percentage, or calc:

""
[..] both values are converted into a ‘calc()’ function that is the sum of a length and a percentage (each possibly zero), and these ‘calc()’ functions have each half interpolated as real numbers.
""

Just looking at the length to percentage case: If I have an interpolation off 20px to 20%, I have to create a calc() function for each of it of the following form?

calc(<length> + <percentage>)

What would be the value for the both calc() functions? What does need to be half interpolated? I must assume that the order (<length> + <percentage> vs <percentage> + <length>) matters in this case, right?

Looking at length to calc: If I have 20px to calc(50% - 20px), how do the interpolation candidates look like?

Thanks a lot for any clarification.

Greetings,
Dirk