Mathematically, the concept of Debt Service Coverage Ratio (DSCR) is simple.
You need:
The property's annual income.
Minus the property's annual operating expenses (and remember that debt service is not an operating expense)
This gives you the Net Operating Income (NOI).
You take the NOI and divide it by the annual debt service (principal and interest payments in year one).
This tells you (and the bank) how many times the property's net income will cover the debt service. This is the DSCR.
If the DSCR is 1.0, the property breaks even after paying the mortgage.
If the DSCR is .80, the property is running at a loss (and the owner/borrower would have to come up with 20% of the debt service out of pocket)
If the DSCR is 1.2, the property covers the debt service and still has a little bit of a buffer.
Most lenders want to see 1.25 or better.
In your scenario, since you were originally looking to use an FHA loan, I assume you don't have a large down payment. That's the problem with "low money down" such as FHA at 3.5% - You are financing 96.5% of the purchase, which makes your debt service much higher than if you put more money down.
So DSCR loans usually don't come with a low down payment. Obviously, it depends primarily on the property's income and expenses (NOI). But qualifying for a DSCR product often means putting more money down, in order lower the cost of the debt service, in order to get the DSCR high enough to satisfy the lender.