No, you can't prorate it.
However, I want to clarify that Social Security is not taxed "at" 85%. Up to 85% of Social Security is taxed at your regular tax rates. For example, if you received $10,000 of Social Security, up to $8500 could be taxable. If you are in the 25% tax bracket, the $8500 of taxable Social Security would result in about $2125 of tax ($8500 x 25%).