Fly sparge efficiency

When fly sparging, you can aim for sparging till the correct volume required or until you get a pre-boil gravity (e.g. 1010).

What happens if you get your volume required but the gravity is still at say 1020?
Does that mean you have a great mash efficiency?
Will there not be more sugar in solution and so a higher OG in the fermenter?

If the opposite happens and you get down to say, 1008 gravity, but you are below your required boil volume, is that poor efficiency?

If you are wanting 1010
and you get 1020 then yes you have over shot your efficiency and have pulled more sugars than you anticipated . which means your temps could be off .
and yes you will have a higher OG since your also going to boil and reduce that amount . It will be even higher.
you can dilute for this problem

If the opposite happens. once again I believe it would be your temps and the types of Mod grains your using.
you can fix that by adding more sugars. adding more grain to the sparge, or using the program here. before you brew to know what you are needing to get to get the best efficiency  out of your system also trail and error

lower than -poor
equal or greater than - your getting good efficiency

you always want to leave that around 25% window in your efficiency for sweetness and flavors
if you did a 100 % efficiency it would not be good at all ....