A deficiency in reading music is easily revealed when a person has not had time to decipher notation in advance. But revealing poor reading skills is not the purpose of sight reading. Several people have remarked that they never had to be taught sight reading. That's because getting the notes right is just reading fluently, you don't have to study "sight reading" as a separate subject for that.
Reading Skills and Sight-Reading Skills are the same skills.
You just have to be much better at it in order to be good at Sight-Reading.
When you are reading music, you should be doing more than recognizing that "This note on the page means I press this key down".
That is a very low level of reading skill.
You also need skill at recognizing groups of notes that form a DbMinor chord so you can immediately play that DbMinor chord instead of thinking separately of the notes Db,F,Ab .... and other things like that.
Actually practicing sight-reading should probably include more than just playing piece after piece which you have never seen before.
It probably should contain "hints" from a qualified instructor as to how you could have done better had you known that certain portions of the score were made up of "chords" and could have been played easier with that knowledge.
And then .... the next piece should contain those same elements to give you an opportunity to practice what you just learned.
Playing random sheets of music is probably less effective.
Of course, any reading helps you get better even if it is presented in a less effective format.
So, if you like to "Sight-Read" .... knock yourself out ... It all helps to improve you skill at playing piano.
BTW .....I am curious about how necessary it is to actually become a good sight-reader.
I would like to see a "show of hands" of how many users have actually been REQUIRED to do sight-reading AT A PROFESSIONAL LEVEL in the last 3 months. I am guessing ... NONE.
I will define REQUIRED to do sight-reading as .....
You were not given access to the notation more than 15 minutes prior to the "GIG".