deficiency in javascript api for creating text track cues

I can create code like this, which works:
var cue1 = new TextTrackCue(3.300,13.130, 'a subtitle');
 cue1.id = 'cue1';
 cue1.line = 8
 cue1.position = 15;
 myTextTrack.addCue(cue1);
 However, if I want to use PERCENTAGES, I have a problem.  I can's say cue1.position = '15%'.  The api doesn't allow it.  I think this is an omission.-- Gideon