at New ilork Ci,111105 Tax Arithmetic Shows Top Rate Is Just a Starter By JACKIE CAUAES WASHINGTON — Despite hints in recent days that President Obama and House Speaker John A. Boehner might compromise on the tax rate to be paid by top earners, a host of other knotty tax questions could still derail a deal to avert a fiscal crisis in January. The math shows why. Even if Republicans were to agree to Mr. Obama's core demand — that the top marginal income rates return to the Clinton-era levels of 36 percent and 39.6 percent after Dec. 31, rather than stay at the Bush-era rates of 33 percent and 35 percent — the additional revenue would be only about a quarter of the $1.6 trillion that Mr. Obama wants to collect over to years. That would be about half of the $800 billion that Republicans have said they would be willing to raise. That calculation alone suggests the scope of the other major tax issues to be negotiated beyond tax rates. And that is why many people in both parties remain unsure that a deal will come together before Jan. 1. Without agreement, more than $5oo billion in automatic tax increases on all Americans and cuts in domestic and military programs will take hold, which could cause a recession if left in place for months, economists say. "The question is making sure that we hit a revenue target that's required for a truly balanced deficit-reduction plan," said Representative Chris Van Hollen of Maryland, the senior Democrat on the House Budget Committee. "And when the president and all of us say this is a question of math, we mean it. It's very hard to make the numbers work without the top rates going back to the full Clinton-en levels." The top tax rates are taking center stage right now because Mr. Obama believes he won a mandate after campaigning relentlessly on the idea of extending Mr. Bush's tax cuts only for households with annual income below $250,000. But the two parties also have ideological differences on taxes affecti