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2 added 239 characters in body
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Try:

I haven't the foggiesthaven't the foggiest as to how he came to this conclusion.

With "as to" you can piece other idioms together, too. There's nothing wrong with I've absolutely no clue as to how he came to this conclusion, either.

Try:

I haven't the foggiest as to how he came to this conclusion.

Try:

I haven't the foggiest as to how he came to this conclusion.

With "as to" you can piece other idioms together, too. There's nothing wrong with I've absolutely no clue as to how he came to this conclusion, either.

1
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Try:

I haven't the foggiest as to how he came to this conclusion.