WebVTT Text Positioning

If I want to position CEA608 text at row 10, column 10 the cue might look
like this:

Cue-8 00:00:52.000 --> 00:00:54.000 position:35% line:58* align:left
size:25% Tis a test

There are 10 characters so the size is 10* cell width (2.5%). Wondering if
I also need to specify the height, which would be one row (5.33%), but I
see no way to do this with the WebVTT spec. Is line height baked into the
implementation?

D