So many people let the fear of the unknown influence their decisions!

Talk to your lender and let them show you the options, then make an informed decision.

Is there a way to calculate the best decision?

You are in a good position now to monitor things in order to make a decision. 

Trying to go for market rent isn't the right play for me but I would give it a $50 bump or so as @Bill Brandt said.Nothing wrong with having the conversation and see how it goes then make decisions after both a conversation and inspection.Also, well done on the analysis. 

You should befriend every single person who is going to have an impact on the decision. 

All of these stories that they had built put them in a position to believe that this project was going to be successful so it's that dream home and here are all the reasons why it makes sense when you're making a business decision you can look at those things and that's nice and it's what you should do and you can even extrapolate you can come to me and say listen here's all the spreadsheets this is why this deal makes sense.And that's a good way to look at it, a business venture but what you must do is contemplate your worst-case scenario if the market corrects back if that asset falls from where it's worth six hundred thousand down to three hundred thousand and believe me no one will believe that can happen but I've seen it so it'll go from six to three.

My wife and I built and purchased our new home last October and we considered renting out our old home, but hadn’t researched it and things were moving so quickly, we didn’t have time to make what was the best decision for our old home and what our needs were for our new home.

It's worth studying data to make sure you're using real numbers and trends to make that decision. 

Even after you own a property you need to constantly do the math because if you don't know how well your property is earning money through current rental income, future rental income and appreciation that results from rental income then their is the chance you will not know how successful you really are and you will not make good decisions based on your lack of information.Always remember that nobody represents you when you purchase a a property.