Tri-X, N+ & N- times

I'd like some advice. I'm taking a class requiring the use of Tri-X and D-76 developer. I've read several articles and books explaining expanded and contracted development, however, none seem to be specific in regards to time or percentage. The normal development time is 10 minutes. Can anyone help me with possible N+1, N+2, N-1, and N-2 times or percentages of basic development times? Thank, Bob

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