@Bill BrandtAt what monthly cash flow did you calculate 12 houses? 

As in, there are a bunch of calculators and Excel spreadsheets out there that will project or estimate if the deal is a good buy.   

I did some more calculations and it seems that I would be able to pay the heloc off  with the extra cash flow if I do the 75%ltv loan at 7.5% interest.

The formula dividing the taxable and non-taxable portions of the principal is not for manual calculation.

I looked into PM services, and based on my calculations, 25% of my net cash flow is going towards PM (factoring their initial 75% one-time setup fee and 8% management).Also, there are no 1-bedroom apartments or condos in the block - only 1+ bedrooms available - not for rent but none present.

Understand the fees involved and calculate the total cost for an entire year of management so you can compare the different managers.

I could use some help to calculate if i'm making a smart investment.

as others have said here CAM costs are traditionally pass throughs that the property owner should be able to show you receipts for the true costs ofGenerally during up front negotiations and LOI's the NNN's mentioned in leases are estimates, but the real NNN's are actually calculated and passed through to the tenants. 

We are unsure of which market we want to invest in yet and are in the research phase and learning how to calculate good properties.

My stance is that I have a standard and ways of calculating my deals that I don't want to be affected by anyone.