Honorable mentions: BRRRR by David Greene and The Book on Estimating Rehab Costs by J Scott

I used the rental calculator and put in the numbers and estimated for management and all, but the analysis showed the mortgage coming in the $600 range, which is over $300 less than what the mortgage should be.

They'll make time for an estimate, and if they're too busy to do the work at the time (which the good ones are) they'll just schedule out the work to when they're not.

Next, discard the rule and use more detailed estimates (which it sounds like you're doing).  

You need to create a spreadsheet either on the old-fashioned spreadsheet paper, with Excel.You need to calculate the profits for hundreds or thousands of properties to determine what your profits will be based on many things I am not going to list because I am on prescribed narcotic, but some of them start with purchase price, rental income profits after all expenses, profits earned from your tenant's help paying down your mortgage, the amount you will earn from expected appreciation (but you can't believe one word anyone tells you about how a property will appreciate, the other 10 or 20 things my brain can't think of and my next most-important thing you (nobody else can or will do this for you).You never estimate how much you earn on a property by listening to anyone because the numbers almost never look pleasant during the period the property is for sale.

In multifamily value-add investments, common risks include:Not being able to achieve target rentsMore tenants moving out than expectedRenovations running behind scheduleRenovation costs exceeding initial estimates (which can be a big deal when you’re renovating hundreds of units)When evaluating deals as potential investments, look for sponsors who have capital preservation of the forefront of the plan and who have a number of risk mitigation strategies in place.

A more accurate estimate would be $10,000 - $15,000 net income for every million in sales, which is hard to reach as a part-timer.There are a lot of variables to consider, but you can expect to make only a couple sales a year and probably less than $10,000 in income.

I’d over estimate at first just to be safe.

I'm having trouble getting an accurate report being my criteria is different from analyzing a new rent house purchase and estimating its cash flow potential.

I think there is a great BP book too, Estimating Rehab costs, by J Scott (I don't have that one yet, but he's a wealth of info, so I'm sure it's great!!)