So get an estimate on flood insurance and factor that into your calculations. 

These builders & flippers, etc are risking their money daily (Plans aligned, calculated) in order to grow personally, mentally, & physically.

If the bill is $100 a month split between four units, increase it to $120 and charge each tenant $30.How to calculate charges.

Combine this website with podcasts, books, and first hand experience (walking properties, calculating cash-flow, making offers, going through closing) you'll get a free education on REI. 

Aparently they calculated it from my 2022 P&L and it results at a quite low LTV.

I would like to find an Excel sheet calculator to use with my hard money cash provider to review the metrics of a deal and something we can both contribute to reviewing the calculations and terms.

You also have to realize that even if you are not using the entire heloc limit "Some" underwriters still calculate the Maximum payment into consideration.There are some ways to pull money out of a Heloc and deposit it into a savings/checking account prior to application to satisfy the PITI reserve requirements.

I have found a number of other users posted this question in the forum over the years but I cannot find a resolution anywhere.The way the Rental Property Calculator derives the "Profit if sold" number seems to have an error in it if you are using the function to increase the rental rate and expenses each year.

Your time is worth more than the time it takes to calculate the interest. 

Here're some online calculators: from Intuit/TurboTax, from H&R Block, from NerdWallet, from TaxAct, and you can find dozens more online.