Variable-width paragraph rules (?)

I've defined my captions for a specific document to include a paragraph rule above or below the caption text (depending on whether the caption appears above or below the graphic). Sometimes I'd like the caption to be one column width, sometimes I'd prefer it to be two column wide, so I need different paragraph styles for each type of caption because I'd like a wider rule for the wider captions (so the rule disappears if I use the same paragraph style in a smaller text frame).

So currently I've got multiple caption styles for above/below, left/right, one column wide/two columns wide. I could eliminate several of these styles if I could just say I want the rule to be, say, 33% of the column width. Is this currently do-able? (I tried entering a percentage -- didn't work). If not, this seems like it would be a nice feature.