My partner and I got dna tests before beginning on the path of procreation. We were making a decision that affected a human being that would have no input on its coming into existence. We owe it to a child we would come to respect and love to be very conscious in every aspect of its life including its genetic viability.

Less than rational thought in such a matter, I believe is very selfish and unkind to a life that I may create.

I understand this isn't the normal way of thinking on the issue, but I wish it was more common.

The premise that backs this approach seems to be that there is a black and white “genetic standard” but the reality is surely very subtle shades of grey. What are the parameters for deciding between genetically viable and genetically unviable?

It's not a genetic standard, it's a phenotypic advantage. Taller humans are preferred by other humans and have a physical advantage over the competition so they are more likely to survive as the fittest.