[JNat's answer](http://anime.stackexchange.com/a/6188/1908) is correct. Just to add a bit of info:

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The variants of "Shibuimaru" that he tries are: 

    渋井丸
    渋伊丸

Note the differences in the middle character.

The variants of "Takuo" that he tries are:

    拓男
    拓夫
    拓雄
    卓男

I think we encounter another minor plot hole here (at least as shown in the anime) - Light stops writing after iterating over all 8 combinations of those names (the first of which, `渋井丸 拓男`, turns out to be correct), when there are actually _at least_ 2 more spellings of "Takuo" (`卓夫` and `卓雄`) that should've been possible.