Someone who is trying to do a BRRRR would need to have an expendable income or enough savings to cover the payments to the lender, insurance, taxes, etc. multiplied by the expected number of months until the rehab is finished, correct? 

But remember, that interest expense deduction is only worth whatever the amount is multiplied by your marginal rate and it only has value if you don’t have other expenses such as depreciation, repairs, etc., or carried forward losses that can offset your revenue from the property.There really ought to be a calculator for this stuff.

There are basic parameters, like "location, location, location", or locking low interest rates is a force multiplier, or materials basically cost the same everywhere but labor expenses are vastly different based upon location.I believe the percentage rules are a little deceiving.

I Multiply after repair value by .53.

I'm thinking I should take the deductible for the flood insurance and multiply it by the annual probability of flooding to get an expected annual cost due to flood damage.

This is really a multiplier, you can do as many as you want for your Magnitude and you have a stable market. 

Another good comparison tool is the Gross Rent Multiplier, GRM. 

We are trained, and use the Gross Rent Multiplier (GRM) when looking at our deals, which is essentially the inverse, or cousin of the 1% calculation.

Time is your greatest resource and your best strength when it comes to multiplying money.

Take that number of needed properties, and multiply by the amount of time you think it will take you to find, fund, buy, rehab (if needed), and fill each of those properties with a tenant.