[nhtool]

I'll usually treat a runner runner nut low draw like a gutshot--the pot has to be around 10 small bets on the flop. But I rarely chase it unless I:

1. have some sort of miracle high draw
2. have counterfeit protection
3. am pretty sure no one else has the nut lo draw

It's important to note that if the pot is big enough to draw to a backdoor low, there's a very good chance at least one other player has a backdoor low, in which case you are drawing to get 1/4 or 1/6...