There are so many moving parts (variables) it will drive you crazy. 

Worked fantastic with 1% fixed rate for 5 years, and 2,5% for the rest of the contract with variable rate. 

Wood has a huge variability in strength--between pine varieties, the stress which can cause them to fail varies by more than 2x and you can see 20%+ increase in strength as it ages.Then I have stress concentrations which will yield and then redistribute the load in practice, but not according to the calculations I'm allowed to do. 

If I hadn’t been an expert in this market there would have been many variables I would have had to estimate which is a rookie mistake.

Slowly build out a repeatable process...if you continue to change things (as in different types of properties, different levels of renovation, different finish material, different neighborhoods, etc.) you create more variables and make it harder to solve to the problem.I am a math guy and less unknowns/variables you have the easier the problem will be to solve. 

Its variable, so they need to see the trend over two years to be able to average out what a monthly average income is for a property. 

It is a rare interest rate environment when you have a HELoC rate sufficiently lower than a fixed-rate mortgage (still likely not lower than a variable rate mortgage ... which are not available in the U.S.A.

Since HELOCs have a variable rate, you’re banking on rates staying the same or decreasing.

Can someone explain the variables that are used to calc the Income-Expense Ratio.

The variables associated with beach STR's can be much more unpredictable.