My initial thought was I'd switch those diaper and formula #s. But that explains it - you said formula was weekly? MAkes more sense.
Diapers should be nowhere near $300/month though. Color me confused... WE paid $100/month for cloth diaper service for 2 kids and I was aware it was much more expensive than just going disposable. (Well much more expensive when we had one in diapers - the 2 were a deal with the service probably).
So my only input is you probably will not need to spend near that much in diapers. Keep in mind for the first few weeks they need something like 10 dispers a day (Each). This goes down considerably with time too (certainly a bigger expense in the very beginning. Not sure how preemies may be different).
Formula is the same way. In a few months they will supplement with other foods and isn't at age 1 when they switch to milk? (Maybe later with preemies?)
Of course this stuff quickly gives way to expenses like activities, daycare/preschool, orthodontia, college (much more in between I am sure). So nothing wrong with setting aside large amounts for that stuff. There will always be something.
In that regards you may be pleasantly surprised with diaper costs, but surprised by other little things.
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Canoeaig - He said $66k taxable income. I think my taxable income is about half what I bring home. He's got 2 new deductions, so explains much.