You are worry free under 20% of AGI.
Above the 20% threshold, there is a 30% threshold, and even a 50% threshold, as mentioned in this public domain article:
"... Only if you contribute more than 20 percent of your adjusted gross income to charity is it necessary to be concerned about donation limits. If the contribution is made to a public charity, the deduction
is limited to 50 percent of your contribution base...
on 20 percent limits and 30 percent limits are ... complicated ... If you give an amount in excess of the applicable limitation... the excess is carried over for the next five years..."
Additionally, if you are going to be subject to the AMT, that introduces another potential issue to your charitable contributions: you may be prevented from realizing full value from those contributions by the AMT being larger than your taxes
would otherwise be without the AMT computation.
The ultimate reference for all of this is IRS Publication 526:
Yahoo Finance makes these points on this subject:
"..In some cases, the IRS won't let you claim all your contributions in one tax year. While generally, your donations cannot be more than 50 percent of your adjusted gross income, although in some instances the limit is 20 percent or 30 percent depending on the type of property you donate and the type of organization to which you give it.
You can carry over your excess contributions for up to five more tax years
, but your carryover amounts will still be subject to the original adjusted gross income limitation rules. For most donors, these limits don't pose a problem. However, the total of all your Schedule A itemized deductions could be reduced if you make a lot of money ($156,400 for 2007 returns for single, married filing jointly
and head of household filers; $78,200 for married couples filing separate returns)...
SOoooo... the simplistic answer to your question is "50%" ... but if you can't take full advantage of the deduction due to (a) AMT or (b) making more than the specified amounts, then the answer "50%" is actually misleading since people usually expect a deduction from substantial contributions.
Hope these thoughts help.