Lactose is not very sweet- try some when you put it in your beer. I would call 8oz the bare minimum for a 5 gallon batch, but I think a pound is more standard for 5 gallons. I would actually start with a pound and adjust down if you think it's too sweet next time. My chocolate stout, which would be sweet without it, uses 12oz.
I can't be arsed to keep up this list of what's in the fermenters, but hey, check out the cool brewery I own!
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