Figuring out my OG

I brewed the Beecave Brewery robust porter two weeks ago. I started using a ph stabilizer and have been getting mad efficiency, like up to 90-93%. My hydrometer broke on brew day and I was unable to get any initial readings. Now that I have my FG can I assume a well attenuated beer and figure my rough OG?
When I first saw my FG reading I thought it was too high, and put the carboy in a warm spot to raise the temp into the mid 70's. Four days later and no change.
Is it safe to assume it is done fermenting, and I simply ended up with a higher gravity porter than intended?
My FG is 1.021, at 75% attenuation (I used Wyeast 1028 London Ale) that would give me an OG of 1.084. Thoughts??

Edit: I brewed with an anticipated efficiency of 75%

I wouldn't trust the number but it will give you a general reference point.

My LHBS owner once suggested that you should keep a small sample out of the fermenter to measure OG if your hydrometer breaks... or get a refractometer :rockin: