Do most people simply use the 50% rule and call it a day?

You simply can't do it.

My offer was simply to let the guy know I was interested and his asking price seemed reasonable enough but nothing is concrete yet.

You have to tie it to an index like a T-bill or the prime rate in the beginning.That’s probably going to affect all but three to five percent of individuals who carry back notes.Remember that these restrictions only apply to seller-carryback transactions on propertiesthat have a dwelling that the buyer will use as their primary residence.

They simply want to say yes, no, or demand changes.

There's many more factors to consider than simply purchase price and rents.

That is simply false. 

See your attorney.While many may think trying to simply cover the buying/borrowing side, the terms can back fire on you later on as a borrower, you might not be dealing with that original lender, so think ahead and long term before going more than 10 years.

I simply am looking to build my network of fellow real estate professionals,

I'm 36 years old, probably should've started this sooner so I don;t have any more time to waste.