Knowing that everyones body and metabolism are unique to them, 3 hours is not the "110% perfect" timing but is just the best and most widely accepted estimate that most of us operate under. So, considering that, I would just co-sign what IainDaniel stated above me.
Relax a little bit. Base your timing on your own trial and error and base your trials and errors on your attempts and results and you can't go wrong. A few minutes in either direction is not going to make or break anyone. Especially not anyone who is closely monitoring their own body.
"Knowing is not enough we must apply." --- "Willing is not enough we must do."