Tim, I don't know that I'd say as a blanket, "just add $600". I think instead I'd try to break it down into the separate components.
For insurance, it sounds like you have a pretty good handle on the costs. I'm running about $1200/year for my 1-2 fams. My 3-unit is covered by the same mortgage and insurance as a 4-unit so I don't know the # for the 3 off the top of my head.
If you're seriously considering a specific building, you can get a quote from an insurance agent for an even more solid #. They'll usually get it back to you within 24 hours, and they're happy to do it since there's a good chance you'll end up giving them the business (principle of reciprocity).
For taxes, look up the assessed value at http://www.statewidemls.com/realtorresources/publicrecords/ and then use the table at http://www.muni-info.state.ri.us/finances/taxrates.php to figure out the yearly and monthly tax amount.
Generally speaking, all your expenses will usually end up being about 40-50% of your income. You can get it lower than 40% if you're super aggressive about keeping costs down and especially if you do some stuff yourself, but in my experience that's a good range to use.
(Note: In my experience, add another 10% to the expenses if you have to pay for the tenants' heat. You'll be amazed at how wasteful people are when they're not paying for it... or maybe you won't be :)
If you want to get a better sense of the other costs, check out http://www.biggerpockets.com/rei/real-estate-property-analysis/ and/or I can also send you an example operating statement if you private msg me your email.
I guess I'd also say, don't get caught up in "analysis paralysis" either. I'm a #s guy too, so I definitely understand where you're coming from, but there's also a point where it's more beneficial to just get started with a decent property, than to run more analyses trying to find the "best" or "most optimal" property.
