I did a 5 gal batch of Orange Vanilla where I used a pound of lactose in it. Turns out that this was not enough. I didn't get a creamy mouthfeel. So If you do I would recomend using 3-4 pounds in a 5 gal batch. This also may sweeten it up a little as it is a sugar, just not as sweet as sugar or honey.
I remember looking up the amounts and this is like 5% of what is in milk. 1 pound to 5 gallons that is. I think a single gallon of milk had like 3 pounds of lactose. If I remember correctly.
I based my recipie off of a cream ale for the amount of Lactose. I forgot to take into account that Beer is actually really thin and less flavorful than Meed, thus the Lactose has less of an effect.
Hope this helps.
I recomend experimenting a bit.
Matrix