The shortcut I always used was to take my effective tax rate from last year's return (i.e. total tax owed / gross income), multiply my self-employment income fo

The shortcut I always used was to take my effective tax rate from last year's return (i.e. total tax owed / gross income), multiply my self-employment income for the past quarter by that rate, maybe round it up somewhat, and pay that. It usually worked out to be close enough that I didn't owe any penalty for underpayment of estimated tax and it was much easier than using the IRS suggested calculation.