No expert but if the instruction say the same time every day then I'd have expected the time when you decide to get up for the day is probably right. I'd be doubting the odd trip to the bathroom during the night would make a lot of difference.
I agree with the others - take it the same time each morning just before you get up.
By the way, we used this method when trying for our youngest and it worked first time. Good luck.
first month ive used it and i can actually work out now when im ovulating. And there are many sites where you can put info in and it will tell you more information on what bits of the chart mean.