IN A LETTER published on this page last week, Pierce Scranton, economic policy director for Mitt Romney’s presidential campaign, took issue with an editorial describing the “implausibility” of Mr. Romney’s tax plan. In the debates and other recent comments, Mr. Romney has taken to suggesting that one way to make his $5 trillion tax cut revenue-neutral would be to “pick a number” — say $25,000 — and let taxpayers take that much, and no more, in deductions.