When to rack to secondary and lactose gravity

Hi all-

I brewed a Chocolate Milk Stout this past Sunday, and the recipe calls for cocoa powder and vanilla bean in the secondary. My question is- when should I rack to the secondary? Beersmith states 4 days primary, then to the secondary, but that seems to be the default. Should I wait till the airlock has stopped bubbling? Wait til it has finished out?

Also, I forgot to take a gravity reading before I added the lactose at near the end of the boil. My OG AFTER the lactose addition was 1.084, which is in the range I wanted. But I am curious how I can calculate backwards
to find what the gravity was before the addition. Can anyone provide any guidance with that?

Thanks for any advice!

Michael
www.thebeerleague.com

The s.g. of lactose is known, as is the volume of the liquid and the amount of lactose added. 
Solving for the s.g. contributed by the lactose is algebra, subtract that from 1.084 and you've got the pre-lactose gravity.

Speaking for the lactose intolerant community, please don't share without telling the lucky person what's in the brew.

Believe me- I know- I am lactose intolerant myself

Thanks for the info - I figured it was a formula but wasn't quite sure of the calculations.

Michael