I've heard about NFCU rates fluctuating with the market.

Builder pricing fluctuates by location and many other factors.

Like you mentioned as well, costs may fluctuate from month to month, and there's some margin of error that you need to account for.

Market fluctuations do not matter.

I see no reason why the payment structure should fluctuate based on the amount of rent that I am getting for the property.

They have fluctuated for a variety of reasons.

Even within those zip codes things fluctuate. 

IMO, using leverage is a matter of timing when you want the income from a portfolio.For example, the more leverage you use, the more your returns will be (assuming you hold long enough to even out fluctuations) because appreciation on your leveraged property will usually provide more wealth than any other element in real estate investing. 

Numbers wise, I use 5% vacancy and capex, maintenance fluctuates between 5 and 10%, and 10% PM fee.

Cambridge has had large fluctuations of population growth over the years but I would say on average it has growth 1%.* Majority of cities had a population growth of about 0.5% in the first half of the 2010s* All cities have a trend of decreasing population growth* Springfield had a small growth in the first half of 2010s (population is 154620 people in 2015) and in the second half of 2010s, its population declined (population 152646 in 2020).* In 2018 population in Newton City and Lower City started to decline.