Just to be clear, you can split the estimated tax payments in whatever way minimizes or eliminates any underpayment penalty; you would split each payment separately (not necessarily the same way for each) since underpayment penalties are calculated on a quarterly basis. What I described in my previous reply is how the previous year's tax separate tax liabilities must be determined for the purpose of the safe-harbor of 100% (or 110%) of the previous year's tax liability.
