How much lactose?

Hello.
I made a pale ale and used Safale US-05 for fermenting. My OG was off before I pitched. I think the problem was with my mill. MY OG was supposed to be 1.059 but I only got 1.045. My FG should be 1.017, but I got 1.003. WAY off! I am going to use lactose to raise the gravity.

Do I add lactose to raise the gravity to 1.017 or should it be a percentage of 1.017? And, how much lactose do I add for 5 gallons?

I was thinking I might be able to use BeerSmith to figure out how much to add, but I do not see lactose as an ingredient.

Thank you in advance.

All-grain or kit?

It was all-grain.

Wow! I did not know that it would matter if it was a kit or all-grain. (goes to show how smart I is not  )
Just for clarification, why does it matter?

The lower OG than expected could be due to the grain crush.  How was everything else on brew day?  Mash temp?  Mash length?  Lautering process?  Those numbers would help eliminate other issues. 

But you got 1.045 OG down to 1.003 FG?  Wow..that is like 93% attenuation, right?  78% AA% would have taken it down to 1.010.  I've read of Safale 05 (same as Wy 1056) being very highly attenuative when fermented warmer than usual.  How does the beer taste at this point? 

I took my mill apart and checked the gap and realized it was tighter than it should have been. So I opened the gap and that might be the cause of the low OG. It has been low before, but not that low. My mash temp and time were as it should have been. I batch sparge, so mashing was as normal, except the numbers.

I used BeerSmith's Refractometer Tool. I set CALCULATION to Fermenting Wort Gravity; Set my refractometer reading to 4.8 brix and my OG at 1.045. That shows a corrected gravity of 1.003. Unless I did it wrong.

I am still cold crashing the beer. I just started crashing it on Saturday morning. I will sit for a week, then I will add CO2. 30 PSI for 2 days then let it sit at normal serving temp for 5 days. So it will be a while before I can REALLY taste the beer. I tried a small sample and it tasted just FINE! I can not wait. I will post more about it when the beer is done.

Garrete said:

I used BeerSmith's Refractometer Tool. I set CALCULATION to Fermenting Wort Gravity; Set my refractometer reading to 4.8 brix and my OG at 1.045. That shows a corrected gravity of 1.003. Unless I did it wrong.

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If your refractometer is calibrated and you got a reading of 4.8 Brix, then I think you did the BeerSmith part correctly.