I would say that you cannot use calculators in poker tournaments and that perhaps the use of a calculator could be considered criminal cheating in the state of Nevada. Over thirty years ago in response to people using hidden computers to count cards at Blackjack Nevada passed a law prohibiting the use of computers in a casino to aid in the beating of a game. Computer use was declared "Cheating" and placed someone doing so under threat of up to five years in prison and up to ten thousand in fines. At the time casino operators being the paranoid lot that they are, or careful business people, did not limit this to blackjack, since they believed they might be able to use it against other forms of cheating, like tracking physical wheels on a slot machine or using a computer to reprogram the newer computer ran slots and other games that were just starting to show up about the same time.
As far as I know they have never used this law to bust a poker player with a calculator, however, rules in a poker room at a poker tournament should conform with the gaming laws. Gaming laws say that the use of a computer to aid in the determination of outcome is illegal (the practicality of prosecuting someone is another story) . Most gaming venues have based their regulation on Nevada gaming law, and most if not all have regulations on the books banning and/or criminalizing the use of computers in a casino to determine outcome. This to me means that the use of a calculator would be banned by default unless their was specifically a rule allowing it. I would sure be comfortable as a tournament director without any rule on the book addressing the use of a calculators, making a ruling that calculators are not allowed