So the key to barriers and any ratings that use them is where a horse is most likely to settle:

* as horse that get back and swoop are neither advantaged nor disadvantages by a wide draw
* On pacers and handy horses need to be able to kick early and thus the more distance they have to travel (both in terms of width of the track and number of runners it must out run) will determine how easy it was for them to settle.

Therefore, how many horses in a field does it take for barriers to matter?

eg: There is no need in a race with 8 horses, IMHO, to bring barriers into the equation.