Sherlock had always been bad at math. Far more than people realized, his was an intuitive science, a place for gut feelings and coming to conclusions later. Many times his deductions were pieced together from data gathered after the fact, after his heart had whispered the answer to him in a voice too strong to ignore. It surprised many to realize that the great consulting detective could barely figure out a train table, much less perform complicated mathematics when it was required of him. If he had time and quiet to work it out, he was capable, but under duress there was no question that he would fail.

He was relieved to find, then, that John Watson far surpassed his skills when it came to calculus and all its assorted ilk. Within a week of living together he was already pressing his taxes upon the beleaguered man, demanding that he figure out the tip when they dined together. Finally there was something that another person could take on for him, something that he could pass off on another without insulting his own talent. This was a relief in itself, but that the man actually seemed honored to do so – well, that just went beyond the pale.

Soon enough John was determining the percentage error when Sherlock performed blood tests and informing him, quietly and quickly, exactly far how a bullet must have been fired to leave a certain pattern of blood spatter on the wall. The radius a man could have ran before they caught him. He never gloated, never lorded it over the detective that he had a skill the other did not; he merely let the facts he gathered speak for themselves. This was a science that Sherlock had never adapted, and that John clearly had; all the more reason to keep him around.

Months passed, and before long it had been nearly a year since Sherlock had seen that beleaguered face and jumped to conclusions, since he had known, in a heartbeat, that John Watson belonged to him. Nothing physical had occurred between them and the doctor was still trying desperately to prove his heterosexuality, half-heartedly flirting with Sarah and taking her on dates. But both knew that he belonged to someone else entirely, as evidenced by the often truncated nature of their dinners, by the fact that John would run halfway across London because of a single text. Soon enough he would have to admit to himself that he was already taken, if not yet by name.

He may not have had the skills to know the percentage of success, but Sherlock was certain, beyond certain, that he could not fail tonight. The champagne, the dinner he had cooked himself (after three tries) and most of all his sincerity as he worked to convince his best friend and partner in crime that they were meant to be together – surely none of this would go to waste.

The door opened and he watched as John's eyes widened in surprise at the meal before them, at the candle light and at Sherlock's winsome smile. He swore he could see excitement shining there as the doctor sat down and stared up at him as if he knew what was about to happen.

Oh yes. The margin of error was very small indeed.