Celtic as far as I can tell simply turned 3,000 seats over to 3,000 standing positions convertible to 3,000 seats for European games. Borussia Dortmund on the other hand have a 1.7 to 1 standing to seated capacity ratio on the Südtribüne, which has more than 24,400 positions in standing room configuration and using that ratio about 14,400 in seated configuration (see
http://www.safestandingroadshow.co.uk/news/borussiadortmundhostsafestandinggroup). My recollection is that back in the 1990s when terraces that were properly inspected and certified under the Safety of Sports Grounds Act were converted directly to seating, the ratio was about 2 to 1 or a touch over; note that the new Kop is physically significantly larger than the old one, which is why the seated capacity is more than half the old one.
One key difference between the Borussia and Celtic installations is that the arrangement at Signal Iduna Park features intermediate risers between each rail, while at Celtic Park they simply installed the rails on the existing double-spaced risers, thereby preventing going beyond the seated capacity. I assume the implication at Borussia is that fans are given a slightly wider space but at half the depth compared to seating configuration.
That's a debate that will presumably be visited in any discussion on licensing standing room in the top leagues in England and Wales. I can see arguments in both directions; the question between a low-density Celtic Park approach and a high density Borussia-type approach may hinge in part on whether the German experience can be replicated in the UK with UK fans.....of course the Celtic Park approach probably wouldn't do much of anything to lower ticket prices, so the case there would depend on stadium atmosphere.