I used a pound of lactose in a Cream Stout with an OG of 1.052 and FG of 1.016 and I think it came out pretty good. I brewed it on 1/4 and just now it's starting to have the creamy mouthfeel I was expecting. Until the point it's at now some bottles had a thinner feel than others, but now they're all consistent and have turned into what I was looking for. As a lot of things go in home brewing, time tends to change things for the better!
I would start with 1lb and see how it turns out, I don't think you'll be disappointed.
I would start with 1lb and see how it turns out, I don't think you'll be disappointed.