Rotten firstly which yeast did you use
Then check to see what attenuation the manufacture states
So let’s assume that it is for want of a better number 77% attenuation
Now if you can get your head around the math side of things
Use the residual of 100% minus 77% = 23%
Take your original gravity say 1047 (use the last two numbers of the reading) and multiply it by .23 to obtain your expected terminal reading
47 x .23= 10.81 or read off hydrometer 1008
Hope that this is not too hard to grasp
Man it took me freaking years of brewing and I am still learning!