The 50 percent rule also includes capital improvements, averaged out over time. A $5,000 heat pump might last 10 years, so on average that's $500 per year. Roofs, hot water heaters, etc. are treated the same way. New houses have an initial round of capital improvements for window coverings, appliances, and landscaping, but the capital improvements tend to be less over the first 10 years or so. I bought several new houses between 1999 and 2002 and I have started the hot water heater replacement cycle. Maintenance costs in general are creeping up. The first A/C unit is probably due in 2014 or 2015, been lucky there so far. Overall, the operating expenses have run significantly less than 50 percent on the newly built houses.
The 50 percent also includes vacancy loss and management. If you manage your properties yourself, you are taking on a job and paying yourself the savings in management fees.
As Arebelspy says, do the research. Come up with what you think your expenses over time will be, factoring in vacancy, capital improvements and professional management. With your high taxes, and the cost of boilers and the like, I would be surprised if you end up with much less than 50 percent.