Related for Bekhorot 121:18

[If an owner] says to his agent, "Go and separate terumah!", he separates an average amount, one fiftieth. [If the owner says] "Go and separate terumah, one fiftieth!" and [the agent inadvertently] reduces it by ten [parts] or increases it by ten [parts], his terumah is [valid] terumah, but if he intended to add [even] one [part more], his terumah is not [valid] terumah. (Ter. 4:4.)