He never was able to use it.

The greatest number of fish he ever caught in one day was three, and you can't add twenty-five per cent.

to three--at least, not in fish.
So he increased his percentage to thirty-three-and-a-third; but that, again, was awkward, when he had only caught one or two; so, to simplify matters, he made up his mind to just double the quantity.
He stuck to this arrangement for a couple of months, and then he grew dissatisfied with it.

Nobody believed him when he told them that he only doubled, and he, therefore, gained no credit that way whatever, while his moderation put him at a disadvantage among the other anglers.

When he had really caught three small fish, and said he had caught six, it used to make him quite jealous to hear a man, whom he knew for a fact had only caught one, going about telling people he had landed two dozen.
So, eventually, he made one final arrangement with himself, which he has religiously held to ever since, and that was to count each fish that he caught as ten, and to assume ten to begin with.