black and tan plan, how to make it work

With St. Patty's day approaching, I'd like to successfully pour some black and tans for my friends. I tried last year, without much success.

I have a smithwick's clone about finished in primary, figuring on using that as my tan. Starting gravity 1.051, finished is 1.011.

How would my stout gravities need to be to get a nice separation between the two? I plan to brew the stout this weekend.

Also, is it the og or fg I need to worry about?

Thanks for any help!