Sell off enough units to recoup expenditure?

If you intend to hold for 3 or 5 or even 7 years, then you are probably overestimating your likely capital expenditures by a significant amount.  

Then add in what the local vacancy rate is say 8% then maybe 10 or 15% for repairs, and at least 5% for capital expenditures. 

It will seem like an enormous, wasteful expenditure at times.

Woody’s largest expenditure in 2004–-$21,515 for workshops and training–-was an educational expense incurred to prepare for a new career, i.e., real estate investor and renter, rather than to maintain or improve skills in an ongoing business or career.

If you make expenditures before you take ownership, this is a gift.  

But sadly, I think its much lower than that:You aren't budgeting for capital expenditures -- you should.

Cash flow positive goal is $400-$600 a month assuming all costs mortgage, taxes, insurance, utilities, cleaning services, vacancies, capital expenditures.- Do it again.

As a little background on myself I am a tax consultant by trade (applying for my CPA license in January), and want to build my real estate portfolio that will provide solid cash-flows to supplement my earnings and accelerate my retirement.How do you estimate operating ans capital expenditures on a month-by-month basis to determine NOI?

Some things to keep in mind are: mortgage (unless all cash purchase), utilities, cap ex (term stands for capital expenditures meaning large ticket items such as roofs, windows, plumbing, etc.) monthly maintenance, HOA (home owners association) if necessary, legal, accounting, vacancy.