Getting back to your original question, it may help to measure your OG before and after you add the lactose. A pound of lactose in 5 gal of wort should add about 6-7 points of gravity. It will also raise your final gravity by about the same amount. For your efficiency calculations, I think you mean your 'mash conversion efficiency'
and if that is what you care about, I would omit the effect of the lactose addition.
Note: most brewing calculators, like the one on Brewer's Friend, can include un-fermentable sugars like lactose in the mash efficiency calculation. All things equal, adding lactose to the formula they use will drive down the reported mash efficiency simply because the sugar in lactose is un-fermentable. So, a little confusing if you're trying to compare your mash process efficiency on a IPA vs. a stout with lactose.