It kinda depends on whose brown malt you're talking about. A friend did some research and sent me this....
A while back I was asking about Baird brown malt, and whether it was really brown and not amber since the sack indicated both "brown" and "amber" (55-70L). Weird. The folks at Baird told me that it could be either brown or amber depending on what the customer wanted it to be, that the two malts are interchangeable. B.S! Anyway, I purchased some Crisp amber malt (35L) from Greg Beron at Culver City Homebrew in So. Cal., and it is visibly identical to Baird. Furthermore, I just received some Thomas Fawcett brown malt from North Country Malt and it is easily, visibly darker then the Baird. I mention this to you because if you're getting your brown malt through the LHBS, and he gets it from Steinbart's, it is Baird amber malt that is mislabeled. If you saw this Thomas Fawcett brown malt, it would be obvious. Makes me mad since I just kegged a porter brewed with the Baird amber, thinking/hoping it was brown malt.
In my bourbon vanilla impy porter recipe, I use 1.5 lb. of the Baird, which is labeled as 70L. That's 8.5 % of the total grist. Whether is really is 70l, I don't know.
Man, I can't believe you kept that info! I'm sure I must have mentioned the obvious taste/roastiness difference between the Baird and the TF.
By the way, John Palmer's San Gabriel Porter recipe is a killer, and uses brown malt.