And is it $3700 - 3900 x 12 months all multipled by 75% for the income calculation ?  

I think at some point if you are the main component to the success, separating yourself or putting yourself in a position to be more advantaged solo is a good idea if calculated in a good way. 

An investor friendly agent can do all sorts of helpful things, like setting you up with a search for properties or connecting you with folks who have recently done deals like the ones you want to do. 6) Consider analyzing a deal or two in your local market, just for practice, with a free use of our Rental Property Calculator (or other calculators).

Calculate the cost of adding the bathroom and how that will translate to increased equity on resale or exit. 

My plan is 3.5~4% downpayment to keep the most cash in hand.I am ready to connect with lenders, but have no idea how to calculate the potential pre-qualified amount. 

There is a easy to use rental calculator up at the top of the page under 'tools'.

Utilize tools such as market data and the BiggerPockets calculator. 

On average I probably net $350-$400/door (that includes my mortgage - I've seen some investors not include their mortgage payment when calculating cash flow). 

I've always calculated NOI as gross income minus operating expenses (not debt or cap ex) period. 

Ask any questions that come up but your best use of time is cost benefit between a second position (blended rate calculation) or a full cash out refinance.