While we lean heavily on the pro's  for repairs and estimates it makes it much easier when we know the typical issues and concerns on the plumbing, roof, electrical etc sides of a property. 

For example, after running comparables, I found that a property's estimated value is $580,000.00, and they have an existing mortgage of $420,000.

Great to see you putting the BP calc to work.Did you get an actual quote for insurance or is this an online estimate?

What are good strategies or online calculators to help make rehab estimates? 

It would also be wise to have a reputable roofing contractor who has been in business for several years walk the roof and give you an estimate.

Most COC estimates are $hit.  

The reason being every site and project is unique, and while I do attempt to provide a reasonable estimate of what it will take to complete your project there are many factors outside my control.

.); the sufficiency of estimated insurance proceeds from ordinance or law insurance and other coverages to repay the Mortgage Loan in the event of partial or full casualty, or condemnation; and for a Tier 3 or Tier 4 Mortgage Loan, if requiring execution of the Limited Payment Guaranty (Form 6020.LPG) would mitigate the risk of the as-rebuilt Property not supporting a Tier 2 Mortgage Loan.

Getting a full, locked loan estimate is the only way to truly compare the two. 

(Typically estimate between 2-3% of the loan amount in closing costs: appraisal, credit report, points/origination, underwriting/processing fees, title fees, prepaid taxes and insurance etc)I am sure there are others I am not remembering