But Mary said no.

"To allow you a full hour to dress," she said, "when half an hour is enough, may answer very well in respect to having you ready for breakfast, but it is no way to cure you of the fault.

That would enable you to play half of the time while you are dressing, without incurring the punishment; but the way to cure you is to make it sure that you will have the punishment to bear if you play at all." So it was decided to allow only half an hour for the dressing-time.
Egbert's mother said she was a little afraid about the one lump of sugar that was left to him when he failed.
"The plan _may_ succeed," she said; "I am very willing that you should try it; but I am afraid that when you are tempted to stop and play in the midst of your dressing, you will say, I shall have _one_ lump of sugar, at any rate, and so will yield to the temptation.

So perhaps it would be safer for you to make the rule that you are not to have any sugar at all when you fail.

Still, _perhaps_ your plan will succeed.