[css3-text] <spacing-limit> incompatible with calc due to different treatment of lengths and %s

The <spacing-limit> type defined for 'word-spacing' in Text 3
<http://dev.w3.org/csswg/css3-text/#spacing-limit> assigns
substantially different meanings to percentages and lengths - the
former sets the word spacing to the given value, while the latter
*increments* the word spacing by the given value.

If calc() is used here and mixes %s and lengths, like "calc(50% +
1ch)", what does this mean?  If I'm reading correctly, I think this
would result in a <length> equal to 1ch + half the default word
spacing, making the total word spacing equal to 1ch + 150% the
default.  Is this intended?

If so, this seems suboptimal, as it's then impossible to, say, use
calc() to set the word-spacing to a particular length.  (I had
expected "calc(0% + 1ch)" to kinda work like that.)  Perhaps we can
alter word-spacing to accept both a percentage and a length, and
combine their effects?

~TJ