The best I can think of is 僕が知る限り + fact + とは限らない. Does this work, are there better alternatives?
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2Are you looking for a plain “according to the information I have”, or more like a “for all I know, he could be dead” kind of thing? There seems to be some variance on what people think the phrase means english.stackexchange.com/questions/92207/… – mirka Nov 10 '15 at 17:30
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1@mirka: the latter. "For all I know, this could've been zombie alien pirates" kind of thing, even. – himself Nov 10 '15 at 20:38
If I understand you correctly, the phrase you're looking for is more about entertaining extreme/absurd possibilities, or humorously saying how little you know about the matter, rather than a sincere “to the extent of my knowledge”.
Variations on “知る限り” will work when you are being sincere, but it doesn't really work in a sarcastic way. I'd say it's closer to a “to my knowledge” or an “as far as I know”:
僕の知る限りではいい人だよ
As far as I know, he's a good person.
When you're just putting out some extreme possibilities, you might consider variations on “可能性もある”. Literally, it means “there is even the possibility that…”, but I think the sentiment can be close:
- 35年という期間は長すぎて、働けなくなる可能性や、それこそ死んでしまう可能性もあります
35 years is such a long time, you could become unable to work, or for all you know you could be dead. - あの人はあまりにもよくテレビに出ていて、毎回 別人の可能性すらある
He's on TV so frequently, for all I know it could be a different person each time. - プロフィールは全くのデタラメの可能性もあるのです。相手が故意にウソをいってだまし、犯罪に巻き込もうとしている可能性さえ考えられます
Profiles can be completely fake. For all you know, the person may be lying to deceive you, trying to involve you in a crime.
(Examples taken and edited from the internet.)
My suggestion is "僕【ぼく】の理解限界【りかいげんかい】[と言【い】えば・について・に関【かん】して]...."
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I would suggest changing 僕【ぼく】 to 自分【じぶん】 -- using 僕【ぼく】 is often inappropriate, depending on the social situation, whereas 自分【じぶん】 ("myself") is more socially neutral. Also, 理解限界【りかいげんかい】 is not really used this way in Japanese. – Eiríkr Útlendi Nov 10 '15 at 19:41