When paying estimated quarterly taxes, can I prorate the amount based on the irregular payment due dates?
Varying the amount to reflect income during the quarter is entirely legitimate -- consider someone like a salesman whose income is partly driven by commissions, and who therefore can't predict the total. The payments are quarterly precisely so you can base them on actual results. Having said that,  I suspect that as long as you show Good Intent they won't quibble if your estimate is off by a few percent. And they'll never complain if you overpay. So it may not be worth the effort to change the payment amount for that last quarter unless the income is very different.