Steven
At a ratio of 2/1 wheat/rye I don't think there's a minimum level. I have
made pleasant breads with 20-40% "non-gluten contributing" grains (rye,
barly, corn) using extremely cheap, white wheat flour with protein as low
as 10.3% and the results were a little heavier than usual but more than
edible.
That said, if you want an open-structured, light loaf you need to be
looking at wheat flour in the 13%+ protein range. The general rule of thumb
is that the higher the protein level the less the flavour of t he finished
bread but, at 1/3 rye, flavour is not going to be a problem<g>.
John