If one participates in an employer sponsored retirement plan and have a personal IRA, how much must one prorate or reduce the contributions that one makes to the personal IRA? Does the phaseout occur if one has a certain level of income?

You may want to check out this publication from the IRS (Publication 590). Your question is covered starting with the section entitled Limit if Covered by Employer Plan. Table 1-2 should have the numbers you are looking for.
IRS Publication 590

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