How much will lactose throw off a hydrometer reading?

I brewed an Oatmeal Sweet Stout and used 1 lb of lactose. I added the lactose at the end of the boil so my OG reading was 1.052. My SG after a week is 1.028. I am using a fairly low attenuating yeast that usually finishes up within 3-5 days and was just wondering if one pound of lactose would keep my gravity that high. I was expecting my FG to be around 1.020. Oh, my fermenting temp is in the low 70's.

Thanks

Let it go two or three more weeks.

A pound of lactose added 6 points to my 5 gallon milk stout.

yogensha said:

A pound of lactose added 6 points to my 5 gallon milk stout.

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that sounds right. since lactose is a sugar, just not fermentable, it should give 'around 30 points per pound per gallon', so yeah, I'd expect it added 1.006 to a 5 gallon batch with 1 lb used.

I'd go for the 3 week primary, no secondary on this one to make sure its finished.